Musical instrument Rob heard (4)
I believe the answer is:
lute
'musical instrument' is the definition.
(lute is a kind of musical instrument)
'rob heard' is the wordplay.
'rob' becomes 'loot' (loot can mean to rob).
'heard' indicates a 'sounds like' (homophone) clue.
'loot' sounds like 'LUTE'.
(Other definitions for lute that I've seen before include "Play instrument" , "Plucked instrument; a sealant" , "Protective packing" , "Elizabethan stringed instrument" , "Old guitar-like instrument played by minstrels" .)