Guy, old, playing popular instrument (8)
I believe the answer is:
mandolin
'instrument' is the definition.
(mandolin is a kind of instrument)
'guy old playing popular' is the wordplay.
'guy' becomes 'man' (I've seen this before).
'playing' indicates anagramming the letters.
'popular' becomes 'in'.
'old' with letters rearranged gives 'dol'.
'man'+'dol'+'in'='MANDOLIN'
(Other definitions for mandolin that I've seen before include "it has strings attached" , "Musical instrument like a small guitar" , "Round-backed musical instrument" , "Stringed instrument, usually played with a plectrum" , "Lute-like instrument" .)