Heroines oddly overlooked for long periods (4)
I believe the answer is:
eons
'long periods' is the definition.
(I've seen this before)
'heroines oddly overlooked' is the wordplay.
'oddly overlooked' means one should take alternating letters (letters in odd-numbered positions removed).
The alternating letters of 'heroines' are 'EONS'.
'for' acts as a link.
(Other definitions for eons that I've seen before include "Nose (anag)" , "Divisions of archaeological time" , "Years and years" , "Very long periods (US)" , "Endless periods" .)