Tail of ruminant grabbed by a smarter animal (8)
I believe the answer is:
anteater
'animal' is the definition.
(anteater is a kind of animal)
'tail of ruminant grabbed by a smarter' is the wordplay.
'tail of' suggests the final letters.
'grabbed by' means one lot of letters goes inside another (inserted letters are grabbed inside).
'smarter' becomes 'neater' ('neat' can be a synonym of 'smart').
The last letter of 'ruminant' is 't'.
'a'+'neater'='aneater'
't' placed inside 'aneater' is 'ANTEATER'.
(Other definitions for anteater that I've seen before include "animal (this one?)" , "Long-snouted insectivore" , "Long tongued creature" , "Insectivorous mammal" , "Pangolin, for example" .)