Doctor joins British crowd (3)
I believe the answer is:
mob
'crowd' is the definition.
(mob can mean a crowd of people)
'doctor joins british' is the wordplay.
'doctor' becomes 'mo' (medical officer).
'joins' is a charade indicator (letters next to each other).
'british' becomes 'b' (abbreviation e.g. in 'BBC').
'mo'+'b'='MOB'
(Other definitions for mob that I've seen before include "Crowd round" , "An unruly or uncontrolled gang" , "Large, disorderly crowd" , "Kind of cap" , "Crime syndicate or disorderly crowd" .)