Be about to happen to the little scamp at last (6)
I believe the answer is:
impend
'be about to happen' is the definition.
(I've seen this before)
'the little scamp at last' is the wordplay.
'the little scamp' becomes 'imp' (I've seen this in another clue).
'at' is a charade indicator (letters next to each other).
'last' becomes 'end' (I've seen this in another clue).
'imp'+'end'='IMPEND'
'to' acts as a link.
(Other definitions for impend that I've seen before include "Be about to take place" , "Be looming" , "Become imminent" , "seen to be around the corner" , "Threaten - be about to happen" .)