Knowing just scamp leaves (3)
I believe the answer is:
sly
'knowing' is the definition.
(sly can mean crafty or knowing)
'just scamp leaves' is the wordplay.
'just' becomes 'simply' (synonyms).
'scamp' becomes 'imp' (synonyms).
'leaves' is a deletion indicator.
'simply' with 'imp' taken out is 'SLY'.
(Other definitions for sly that I've seen before include "Doing things in unpleasant and secret way" , "Devious and furtive" , "'Crafty, cunning (3)'" , "'Cunning, wily (3)'" , "Crafty, devious" .)