Quick to knock with identification (5)
I believe the answer is:
rapid
'quick' is the definition.
(synonyms)
'knock with identification' is the wordplay.
'knock' becomes 'rap' (synonyms).
'with' is a charade indicator (letters next to each other).
'identification' becomes 'ID' (abbreviation).
'rap'+'id'='RAPID'
'to' acts as a link.
(Other definitions for rapid that I've seen before include "Fast, quick" , "Precipitate" , "Happening at great speed" , "Quickly accomplished" , "Fleet, swift" .)