Disguise the nature of feminine opinion (6)
I believe the answer is:
belief
'opinion' is the definition.
(an opinion is a personal belief)
'disguise the nature of feminine' is the wordplay.
'disguise the nature of' becomes 'belie' (to belie is to misrepresent or disguise).
'feminine' becomes 'f' (abbreviation listed in Chambers).
'belie'+'f'='BELIEF'
(Other definitions for belief that I've seen before include "Conviction, faith" , "Credo" , "Tenet, creed" , "Religious conviction" , "Firm conviction" .)