Visit doctor and mostly express one's personal belief (4,2)
I believe the answer is:
drop in
'visit' is the definition.
(to drop in on someone is to visit them)
'doctor and mostly express one's personal belief' is the wordplay.
'doctor' becomes 'Dr' (abbreviation).
'and' says to put letters next to each other.
'mostly' means to remove the last letter.
'express one's personal belief' becomes 'opine' (to opine is to share an opinion).
'opine' with its final letter taken away is 'opin'.
'dr'+'opin'='DROP IN'
(Other definitions for drop in that I've seen before include "Pay casual call or visit" , "Make a casual call" , "Arrive for casual visit" , "Pay a casual visit - by parachute" , "Pay a casual visit unannounced" .)