Musical instrument found in sack, did you say? (4)
I believe the answer is:
lute
'musical instrument' is the definition.
(lute is a kind of musical instrument)
'sack did you say?' is the wordplay.
'sack' becomes 'loot' (I've seen this before).
'did you say?' shows a homophone (sound like).
'loot' sounds like 'LUTE'.
'found in' is the link.
(Other definitions for lute that I've seen before include "Protective packing" , "Subject to drugs" , "Old plucked instrument" , "clay" , "Ancient stringed instrument" .)