Decisive blow spinning ruminant round (4)
I believe the answer is:
kayo
'decisive blow' is the definition.
(a KO, a knock-out)
'spinning ruminant round' is the wordplay.
'spinning' shows that the letters should be reversed in order.
'ruminant' becomes 'yak' (yak is a kind of ruminant).
'round' becomes 'o' (round can mean a round object).
'yak' back-to-front is 'kay'.
'kay'+'o'='KAYO'
(Other definitions for kayo that I've seen before include "Knock out in boxing" , "floor" , "Stun" , "Knockout" , "Decisive blow (in boxing)" .)