Visit doctor and say what you think, briefly (4,2)
I believe the answer is:
drop in
'visit' is the definition.
(dropping in is a kind of visiting)
'doctor and say what you think briefly' is the wordplay.
'doctor' becomes 'Dr' (abbreviation).
'and' says to put letters next to each other.
'say what you think' becomes 'opine' (to give an opinion).
'briefly' means to remove the last letter.
'opine' with its final letter removed is 'opin'.
'dr'+'opin'='DROP IN'
(Other definitions for drop in that I've seen before include "Briefly visit" , "Arrive for casual visit" , "Pay a casual visit unannounced" , "Visit informally" , "Surgery held by doctor" .)