Small stuff? Beat it! (5)
I believe the answer is:
scram
'beat it' is the definition.
(I've seen this before)
'small stuff?' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'stuff?' becomes 'cram' (cramming is a kind of stuffing).
's'+'cram'='SCRAM'
(Other definitions for scram that I've seen before include "Scarper" , "Leave hurriedly" , "Hop it!" , "on your bike!" , "Buzz off!" .)