Sorry, but I think I have to do frame challenge on the whole question, before speaking about organizing defenses, as it appears to me that some of your premises don't work together, unless you are willing to do some heavy handwaving or magic. Some of the issues were raised by @AlexP 's answer, but in addition:
1) Not only humans have no "breeding season", but it doesn't make much evolutionary sense that they should. Humans have the longest childhood of all known Earth animals. The only comparable animal is elephant. All the others reach maturity at least 5x times faster -- generally, a couple of years, max.
During this time children are vulnerable and weak. If there is a "breeding season", it means all children are born at approximately same time, and this incurs heavy penalty on the amount of women available to care for anything else. Of course, the same goes for the period of pregnancy.
Next, having all children of the same age around, heavily increases chances of a generation dying all together. For example, a heavy drought. Or a flood. Epidemy. Children of younger ages are much more susceptible to being killed by things that larger and older kids will survive.
2) Heavy work. Ancient world presumes most of work is hand-work. While females and males both can be assumed stronger physically than current day ones, in general, males of the human species can be expected to have about two times stronger upper body and up to about 1/3d stronger lower body. Whatever the settlement a clan lives in, what will they do, when they urgently need some wall repaired against predators, or a burst dam? Send for the nearest roving band, which might be a week away or more, when not "in breeding season"? Assigning twice as many females to the task, as the "surplus" men? Quite possible, but not very viable by survival economics -- in ancient times, and not only then -- there frequently was little enough food to go around. Add to it the "breeding season", and you really have a recipe for disaster: a lightning strike sets your settlement on fire, there have been hungry wolves spotted in vicinity, and half of your women are in the last months of pregnancy.
Due to (1) and (2) your premise of: "Boys exit their clans upon reaching adulthood. These males leave their families to join small bands of males who live as nomads. Roving bands of these men wander the countryside, hunting and living off the land" appears to place incredible burden on the women. Instead of having some division of work and the males at least regularly bringing back valuable sources of protein and fat, not to talk about doing the heavy lifting, you have males barely surviving in wilderness -- very much dependant on their luck, while women have to assign disproportionate amount of, well, women, doing the work that males could do faster/easier.
This separate living sounds like a recipe for extinction.
I am not even starting to consider that ancient communities were generally very wary and hostile of strangers, as compared to members of one's own clan. While you call your society "clans", what it amounts basically is men being expulsed from society, to form their own. What kind of loyalty could they be expected to have towards the women settlement then?