Female isn't weak (5)
I believe the answer is:
faint
'weak' is the definition.
(I know that weak can be written as faint)
'female isn't' is the wordplay.
'female' becomes 'f' (common abbreviation).
'isn't' becomes 'aint' (non-standard version of 'isn't').
'f'+'aint'='FAINT'
(Other definitions for faint that I've seen before include "Weak" , "Swoon, pass out" , "Swoon, lose consciousness" , "Pale - pass out" , "Lacking clarity" .)