A quick blow to avoid finally (5)
I believe the answer is:
rapid
'a quick' is the definition.
(I've seen this before)
'blow to avoid finally' is the wordplay.
'blow' becomes 'rap' (rap is a kind of blow).
'to avoid finally' becomes 'id' (I can't explain this - if you can you should give a lot more credence to this answer).
'rap'+'id'='RAPID'
Can you help me to learn more?
(Other definitions for rapid that I've seen before include "Fast, quick" , "Happening at great speed" , "Happening quickly" , "moving fast" , "Flying" .)