Doctor joins bishop in crowd (3)
I believe the answer is:
mob
'crowd' is the definition.
(mob can mean a crowd of people)
'doctor joins bishop' is the wordplay.
'doctor' becomes 'mo' (medical officer).
'joins' is a charade indicator (letters next to each other).
'bishop' becomes 'B' (abbreviation for bishop in chess).
'mo'+'b'='MOB'
'in' acts as a link.
(Other definitions for mob that I've seen before include "Crowd round" , "Unruly lot" , "Ugly crowd" , "Kind of cap" , "Large, disorderly crowd" .)