Doctor's operation involves part of the body (5)
I believe the answer is:
torso
'part of the body' is the definition.
('torso' is part of 'body')
'doctor's operation involves' is the wordplay.
'involves' indicates a hidden word.
'TORSO' is hidden within 'doctors operation'.
(Other definitions for torso that I've seen before include "Piece of bronze" , "Body, minus its head and limbs" , "Trunk; headless, limbless body" , "upper body" , "unfinished work" .)