The hi-fi's 'noise gremlin'? Nonsense! (6)
I believe the answer is:
humbug
'nonsense' is the definition.
(I know that humbug is a type of nonsense)
'the hi-fi's 'noise gremlin'?' is the wordplay.
I cannot really see how this works, but
''noise' could be 'hum' (hum is a kind of noise) and 'hum' is present in the answer.
The remaining letters 'bug' 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 humbug that I've seen before include "Boiled sweet - rubbish" , "Con" , "Baloney" , "Hypocrite - bah!" , "Minty sweet" .)