Firstly there are better things to take as anti-oestrogens than melatonin. But that's not for me to recommend or comment on really.(well maybe later)
Extended use will probably cause hormone imbalances in women as it effects progesterone as well.
As I said before it has a general role in reducing oestrogen in the body, how it does this is it reduces the amount of cellular receptors. It inhbits the activity of some types of cancer tumours for this reason, it complicated but there is a direct relationship between melatonins activity and inhibition of oestrogen response pathways.
Anyway it does aid in sleep, but thats pretty well known. Bodybuilding and particularly overtraining can reduce levels so supplementation may be beneficial. Disrupted sleep patterns may result in elevated oestrogen which melatonin may have a positive effect on.
'Theoretically' Clomid would be far more effective. It acts by blocking oestrogen receptors at the hypothalamus and pituitary in more specific manner.
Also some cruciferous vegetables contain indoles which has anti oestrogenic effects. Also reduce oestrogen by avoiding drinking tap water.