Instrument jams - it's rubbish (12)
I believe the answer is:
fiddlesticks
'rubbish' is the definition.
(both can mean nonsense)
'instrument jams it's' is the wordplay.
I cannot really see how this works, but
'instrument' could be 'fiddle' (fiddle is a kind of instrument) and 'fiddle' is found in the answer.
'it' could be 't' (abbreviation. e.g. in 'tis) and 't' is located in the answer.
The remaining letters 'sicks' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for fiddlesticks that I've seen before include "Bother!" , "tripe" , "Rhubarb" , "Violin bows - rubbish!" , "Blast!" .)