Delivery post procedure subject to doctor (4,4)
I believe the answer is:
mail drop
'delivery' is the definition.
(I've seen this before)
'post procedure subject to doctor' is the wordplay.
I cannot really understand how this works, but
'post' could be 'mail' (synonyms) and 'mail' is located in the answer.
'procedure' could be 'op' (short for surgical operation) and 'op' is present in the answer.
'doctor' could be 'dr' (abbreviation) and 'dr' is found in the answer.
No letters remain.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for mail drop that I've seen before include "post finally obtained here in America?" , "Postal delivery" , "post-delivery" , "Letters" .)