Yes, Jack, it's a musical instrument (5)
I believe the answer is:
sitar
'a musical instrument' is the definition.
(sitar is a kind of musical instrument)
'yes jack' is the wordplay.
'yes' becomes 'si' (I can't justify this - if you can you should believe this answer much more).
'jack' becomes 'tar' (both are informal terms for sailor).
'si'+'tar'='SITAR'
'it's' acts as a link.
Can you help me to learn more?
(Other definitions for sitar that I've seen before include "Indian string instrument" , "Astir with Indian long-necked lute" , "Guitar-like instrument" , "instrument out east" , "Lasting from morning to night" .)