Quickly knock but to no avail (7)
I believe the answer is:
rapidly
'quickly' is the definition.
(I know that quickly can be written as rapidly)
'knock but to no avail' is the wordplay.
I cannot quite see how this works, but
'knock' could be 'rap' (synonyms) and 'rap' is found within the answer.
'to no avail' could be 'idly' (I've seen this in another clue) and 'idly' is found within the answer.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for rapidly that I've seen before include "At a great rate" , "With great speed" , "Quickly" , "Speedily" , "right off the bat" .)