Chap wearing trousers playing old musical instrument (8)
I believe the answer is:
mandolin
'musical instrument' is the definition.
(mandolin is a kind of musical instrument)
'chap wearing trousers playing old' is the wordplay.
'chap' becomes 'Man' (chap can mean a man).
'wearing' becomes 'in' (as in being 'in' certain clothes).
'trousers' indicates putting letters inside (to trouser can mean to steal or take).
'playing old' becomes 'dol' (I can't justify this - if you can you should believe this answer much more).
'man'+'in'='manin'
'manin' placed around 'dol' is 'MANDOLIN'.
(Other definitions for mandolin that I've seen before include "it has strings attached" , "one's deployed in the kitchen" , "Stringed instrument, usually played with a plectrum" , "Small guitar-like instrument" , "vegetable slicer" .)