Cracking doctor's surgery is in decline (5)
I believe the answer is:
drops
'is in decline' is the definition.
I can't tell whether this definition defines the answer.
'cracking doctor's surgery' is the wordplay.
'cracking' is an insertion indicator (inserted letters go into a gap or crack).
'doctor's' becomes 'drs' (abbreviation).
'surgery' becomes 'op' (short for operation).
'drs' enclosing 'op' is 'DROPS'.
Can you help me to learn more?
(Other definitions for drops that I've seen before include "Relinquishes" , "Falls" , "Abandons" , "sweets" , "Lets fall" .)