Animal: one leapt out (8)
I believe the answer is:
antelope
'animal' is the definition.
(thesaurus)
'one leapt out' is the wordplay.
'out' is an anagram indicator (out can mean wrong or inaccurate).
'one'+'leapt'='oneleapt'
'oneleapt' anagrammed gives 'ANTELOPE'.
(Other definitions for antelope that I've seen before include "Hartebeest, perhaps" , "Eg, gazelle or gnu" , "horny beast!" , "A graceful gazelle from Africa" , "Kind of swift-running deer" .)