Some tiger, a terrifying animal (3)
I believe the answer is:
rat
'animal' is the definition.
(rat is a kind of animal)
'some tiger a terrifying' is the wordplay.
'some' indicates the answer is hidden within the clue (some parts of the word are used).
'RAT' can be found hidden inside 'tiger a terrifying'.
(Other definitions for rat that I've seen before include "Sewer rodent" , "one who betrayed" , "Strike-breaker" , "One deserting sinking ship" , "one who deserts the sinking ship?" .)