Perhaps sexist one when in bed (6)
I believe the answer is:
biased
'perhaps sexist' is the definition.
'one when in bed' is the wordplay.
'one' becomes 'i' (Roman numeral).
'when' becomes 'as'.
'in' is an insertion indicator.
'i'+'as'='ias'
'ias' placed within 'bed' is 'BIASED'.
(Other definitions for biased that I've seen before include "Prejudiced or influenced in favour or against" , "Prejudiced or partisan" , "Favouring one person or side over another" , "Not impartial" , "Having favourite" .)