Fast to knock with identification (5)
I believe the answer is:
rapid
'fast' is the definition.
(similar in meaning)
'knock with identification' is the wordplay.
'knock' becomes 'rap' (synonyms).
'with' says to put 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 "Quick or swift" , "Precipitate" , "Prompt" , "Happening quickly" , "Electrical connector" .)