Be against top model going topless (6)
I believe the answer is:
oppose
'be against' is the definition.
('oppose' can be a synonym of 'be against')
'top model going topless' is the wordplay.
'model' becomes 'pose' ('pose' can be a synonym of 'model').
'going topless' means to remove the first letter.
'top'+'pose'='toppose'
'toppose' with its first letter removed is 'OPPOSE'.
(Other definitions for oppose that I've seen before include "Resist, be hostile to" , "Obstruct, resist" , "'Refute, stand against (6)'" , "match" , "Resist, contend with" .)