Doctor joins British horde (3)
I believe the answer is:
mob
'horde' is the definition.
(I've seen this before)
'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 "Throng" , "Ugly crowd" , "Crowd round" , "press round" , "An undisciplined gang of people" .)