Let go: doctor needs small operation (4)
I believe the answer is:
drop
'let go' is the definition.
(thesaurus)
'doctor needs small operation' is the wordplay.
'doctor' becomes 'dr' (abbreviation).
'needs' means one lot of letters go next to another.
'small operation' becomes 'op' ('small' suggests a short form).
'dr'+'op'='DROP'
(Other definitions for drop that I've seen before include "Small quantity of liquid" , "Pendant earring" , "Fail to hold" , "Collapse, globule" , "Small bit - fall" .)