Black bird departs flock (5)
I believe the answer is:
'flock' is the definition.
(I've seen this before)
'black bird departs' is the wordplay.
'black bird' becomes 'crow' (I've seen this before).
'departs' becomes 'd' (common abbreviation e.g. train timetables).
(Other definitions for crowd that I've seen before include "Spectators" , "Large number of people gathered together" , "Large group of people" , "Masses" , "assembly" .)