Instrument that's round? Gong's round (4)
I believe the answer is:
oboe
'instrument' is the definition.
(type of musical instrument)
'round? gong's round' is the wordplay.
'round?' becomes 'o' (round can mean a round object).
'gong' becomes 'obe' (Order of the British Empire award or 'gong').
'round' is an insertion indicator.
'o' going inside 'obe' is 'OBOE'.
'that's' is the link.
(Other definitions for oboe that I've seen before include "music producer" , "Woodwind instrument of treble pitch" , "Orchestral instrument" , "Musical instrument" , "Orchestral piece" .)