Small enthusiastic crowd (5)
I believe the answer is:
swarm
'crowd' is the definition.
(I know that swarm is a type of crowd)
'small enthusiastic' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'enthusiastic' becomes 'warm' (I've seen this before).
's'+'warm'='SWARM'
(Other definitions for swarm that I've seen before include "Army" , "Body of bees" , "Horde" , "some setting up colony" , "Large crowd" .)