Instrument puts old man in confusion (8)
I believe the answer is:
mandolin
'instrument' is the definition.
(mandolin is a kind of instrument)
'old man in confusion' is the wordplay.
'confusion' indicates an anagram.
'old'+'man'+'in'='oldmanin'
'oldmanin' is an anagram of 'MANDOLIN'.
'puts' acts as a link.
(Other definitions for mandolin that I've seen before include "'Captain Corelli's - - '" , "Musical instrument like a small lute" , "Musical instrument like a small guitar" , "Almond in the stringed instrument, oddly" , "Round-backed musical instrument" .)