Ruminant: one leapt out (8)
I believe the answer is:
antelope
'ruminant' is the definition.
(antelope is a kind of ruminant)
'one leapt out' is the wordplay.
'out' indicates an anagram (out can mean wrong or inaccurate).
'one'+'leapt'='oneleapt'
'oneleapt' anagrammed gives 'ANTELOPE'.
(Other definitions for antelope that I've seen before include "One petal for this animal" , "bounder!" , "One leapt badly on the graceful animal" , "Springbok or gnu?" , "Springboks, say" .)