Rascal to knock with spring onion (11)
I believe the answer is:
rapscallion
'rascal' is the definition.
(synonyms)
'knock with spring onion' is the wordplay.
'knock' becomes 'rap' (synonyms).
'with' means one lot of letters go next to another.
'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" , "Disreputable person - local sprain (anag)" , "Scoundrel" , "Terror" , "mischief maker" .)