Best work - it's about gender mostly (7)
I believe the answer is:
optimal
'best' is the definition.
('optimal' can be similar in meaning to 'best')
'work it's about gender mostly' is the wordplay.
'work' becomes 'op' (abbreviation for opus**).
'about' says the letters should be written backwards.
'gender' becomes 'male' (I have seen 'A gender ' mean 'male' so perhaps 'gender' could also mean 'male').
'mostly' means to remove the last letter.
'it' backwards is 'ti'.
'male' with its final letter removed is 'mal'.
'op'+'ti'+'mal'='OPTIMAL'
(Other definitions for optimal that I've seen before include "top, surprisingly" , "Most favouring" , "In the best condition" , "couldn't be better" , "Plot aim (anag) - most favourable" .)