Quick knock I had (5)
I believe the answer is:
rapid
'quick' is the definition.
('rapid' can be a synonym of 'quick')
'knock i had' is the wordplay.
'knock' becomes 'rap' ('rap' can be a synonym of 'knock').
'i had' becomes 'id' (I'd).
'rap'+'id'='RAPID'
(Other definitions for rapid that I've seen before include "Electrical connector" , "Fast, quick" , "Brisk" , "Speedy, quick" , "moving fast" .)