Small crowd get lost (5)
I believe the answer is:
scram
'get lost' is the definition.
(thesaurus)
'small crowd' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'crowd' becomes 'cram'.
's'+'cram'='SCRAM'
(Other definitions for scram that I've seen before include "Leave quickly" , "Scarper" , "Shoo!" , "make emergency shut-down" , "Slangy order to go away, beat it" .)