Cease to use doctor before operation (4)
I believe the answer is:
drop
'cease' is the definition.
(I know that drop is a type of cease)
'doctor before operation' is the wordplay.
'doctor' becomes 'dr' (abbreviation).
'before' says to put letters next to each other.
'operation' becomes 'op'.
'dr'+'op'='DROP'
'to use' acts as a link.
(Other definitions for drop that I've seen before include "Unload" , "to omit the > smallest amount" , "Dispense with - sink" , "Spot" , "Collapse, globule" .)