Sack medic before surgery (4)
I believe the answer is:
drop
'sack' is the definition.
(I've seen this before)
'medic before surgery' is the wordplay.
'medic' becomes 'dr' (doctor).
'before' is a charade indicator (letters next to each other).
'surgery' becomes 'op' (short for operation).
'dr'+'op'='DROP'
(Other definitions for drop that I've seen before include "A little medicine" , "See 1" , "Bit of water" , "down, say, casually" , "Leave out of team" .)