Get lost with small pack (5)
I believe the answer is:
scram
'get lost' is the definition.
(interjections with similar meaning)
'with small pack' is the wordplay.
'with' is a charade indicator (letters next to each other).
'small' becomes 's' (abbreviation - e.g. clothes size).
'pack' becomes 'cram' (synonyms).
's' put next to 'cram' is 'SCRAM'.
(Other definitions for scram that I've seen before include "Make oneself scarce" , "Shoo!" , "Buzz off!" , "Go away quickly" , "Slangy order to go away, beat it" .)