Not infrequently, a wrong note comes out loud! (5)
I believe the answer is:
often
'not infrequently' is the definition.
('often' is the opposite to 'infrequently')
'a wrong note comes out loud' is the wordplay.
I cannot quite see how this works, but
'note' could be 'te' (musical note) and 'te' is found in the answer.
'loud' could be 'f' (short for forte - 'loud' in music) and 'f' is present in the answer.
The remaining letters 'on' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for often that I've seen before include "Turn away" , "Frequent" , "A great deal" , "A lot" , "More than occasionally" .)