Pelvis operation reversed for large mammal (5)
I believe the answer is:
hippo
'large mammal' is the definition.
(I've seen this before)
'pelvis operation reversed' is the wordplay.
'pelvis' becomes 'hip' (synonyms).
'operation' becomes 'op'.
'reversed' says the letters should be written in reverse.
'op' backwards is 'po'.
'hip'+'po'='HIPPO'
'for' acts as a link.
(Other definitions for hippo that I've seen before include "Aquatic mammal" , "Wallowing creature" , "Beastly sort" , "22 aquatic animal" , "Massive thick-skinned animal of African rivers" .)