Crowd beginning to build after short time (3)
I believe the answer is:
mob
'crowd' is the definition.
(mob is a kind of crowd)
'beginning to build after short time' is the wordplay.
'beginning to' says to take the initial letters.
'after' is a charade indicator (letters next to each other).
'short time' becomes 'mo' (slang abbreviation for 'moment').
The initial letter of 'build' is 'b'.
'b' put after 'mo' is 'MOB'.
(Other definitions for mob that I've seen before include "Throng" , "An unruly or uncontrolled gang" , "Rabble" , "An undisciplined gang of people" , "Unruly lot" .)