Sink for doctor before surgery (4)
I believe the answer is:
drop
'sink' is the definition.
('drop' can be a synonym of 'sink')
'doctor before surgery' is the wordplay.
'doctor' becomes 'dr' ('Dr' can be a synonym of 'doctor'**).
'before' is a charade indicator (letters next to each other).
'surgery' becomes 'op' (short for operation).
'dr'+'op'='DROP'
'for' acts as a link.
(Other definitions for drop that I've seen before include "Wee dram" , "Plummet" , "Tiny bit of liquid" , "Desert" , "Discontinue; sink" .)