Cheers one knockout Japanese drum (5)
I believe the answer is:
taiko
'japanese drum' is the definition.
(I've seen this before)
'cheers one knockout' is the wordplay.
I cannot really see how this works, but
'cheers' could be 'ta' (I've seen this in other clues) and 'ta' is found in the answer.
'one' could be 'i' (Roman numeral) and 'i' is found within the answer.
'knockout' could be 'ko' ('KO' can be a synonym of 'knockout') and 'ko' is found in the answer.
No letters remain.
This may be the basis of clue (or it may be nonsense).
(Another definition for taiko that I've seen is " Japanese drum".)