Mostly get on well around popular medical centre (6)
I believe the answer is:
clinic
'medical centre' is the definition.
(a clinic is a centre for medical treatment)
'mostly get on well around popular' is the wordplay.
'mostly' means to remove the last letter.
'get on well' becomes 'click'.
'around' means one lot of letters goes inside another.
'popular' becomes 'in' ('in' can mean trendy).
'click' with its final letter taken off is 'clic'.
'clic' placed around 'in' is 'CLINIC'.
(Other definitions for clinic that I've seen before include "Place for medical treatment or political advice" , "Place for specialised treatment, from doctor or politician" , "Surgery" , "Healthcare facility" , "Building where outpatients are treated" .)