Stringed instrument in sack, we hear (4)
I believe the answer is:
lute
'stringed instrument' is the definition.
(lute is a kind of stringed instrument)
'sack we hear' is the wordplay.
'sack' becomes 'loot' (I've seen this before).
'we hear' shows a homophone (sound like).
'loot' sounds like 'LUTE'.
'in' acts as a link.
(Other definitions for lute that I've seen before include "Protective packing" , "instrument that minstrel played" , "Stringed instrument shaped like a halved egg" , "Old stringed instrument with a pear-shaped body" , "Ancient stringed instrument" .)