Quickly knock for no particular reason (7)
I believe the answer is:
rapidly
'quickly' is the definition.
(synonyms)
'knock for no particular reason' is the wordplay.
'knock' becomes 'rap' (synonyms).
'for no particular reason' becomes 'idly' (I've seen this in another clue).
'rap'+'idly'='RAPIDLY'
(Other definitions for rapidly that I've seen before include "right off the bat" , "Speedily" , "Quickly" , "With great speed" , "At a great rate" .)