Laugh loudly — quite a fit! (4,5)
I believe the answer is:
fall about
'laugh' is the definition.
(as in falling about laughing)
'loudly quite a fit' is the wordplay.
I cannot really see how this works, but
'loudly' could be 'f' (short for forte - 'loud' in music) and 'f' is found within the answer.
'quite' could be 'all' (I've seen this in another clue) and 'all' is located in the answer.
'a' is found in the answer.
'fit' could be 'bout' (I've seen this in other clues) and 'bout' is present in the answer.
This accounts for all the letters.
This explanation may well be incorrect...
(Other definitions for fall about that I've seen before include "Laugh heartily" , "Laugh uncontrollably" , "Laugh a lot" .)