The opposite of legs being trim (4)
I believe the answer is:
snip
'trim' is the definition.
(I've seen this before)
'the opposite of legs' is the wordplay.
'the opposite of' is a reversal indicator.
'legs' becomes 'pins' (pin is a kind of leg).
'pins' written backwards gives 'SNIP'.
'being' acts as a link.
(Other definitions for snip that I've seen before include "Cut with shears" , "Bargain (coll)" , "nick)" , "Something cheap or easy to do" , "Cut with scissors in small strokes" .)