Disheartened rabbit - one subdued by monster (6)
I believe the answer is:
bunyip
'monster' is the definition.
(I've seen this before)
'disheartened rabbit one subdued by' is the wordplay.
'disheartened rabbit' becomes 'buny' (bunny without n).
'one' becomes 'i' (Roman numeral).
'subdued' becomes 'p' (p for piano and quiet).
'by' is a charade indicator (letters next to each other).
'buny'+'i'+'p'='BUNYIP'
(Other definitions for bunyip that I've seen before include "Australian legendary swamp-monster" , "monstrous Australian" , "monster" .)