Imp to knock with spring onion (11)
I believe the answer is:
rapscallion
'imp' is the definition.
(I know that imp can be written as rapscallion)
'knock with spring onion' is the wordplay.
'knock' becomes 'rap' (synonyms).
'with' is a charade indicator (letters next to each other).
'spring onion' becomes 'scallion' (I've seen this before).
'rap'+'scallion'='RAPSCALLION'
'to' is the link.
(Other definitions for rapscallion that I've seen before include "Plain carols (anag) - rogue" , "Scoundrel" , "Monkey" , "Disreputable person - local sprain (anag)" , "Terror" .)