Declines operation in doctor's (5)
I believe the answer is:
drops
'declines' is the definition.
(dropping is a kind of declining)
'operation in doctor's' is the wordplay.
'operation' becomes 'op'.
'in' is an insertion indicator.
'doctor's' becomes 'drs' (abbreviation).
'op' put into 'drs' is 'DROPS'.
(Other definitions for drops that I've seen before include "Falls" , "Small quantities of liquid" , "sweets" , "Abandons" , "Gives up" .)