Decline surgery overseen by doctor? (4)
I believe the answer is:
drop
'decline' is the definition.
(dropping is a kind of declining)
'surgery overseen by doctor?' is the wordplay.
'surgery' becomes 'op' (short for operation).
'overseen by' is a charade indicator (letters next to each other) (in a down clue, some letters go above others).
'doctor?' becomes 'dr' (abbreviation).
'op' put after 'dr' is 'DROP'.
(Other definitions for drop that I've seen before include "Fall; delivery" , "Fall down with liquid globule" , "Shoot" , "Fail to hold" , "Dispense with - sink" .)