Giant cat biting a dog's tail (5)
I believe the answer is:
magog
'giant' is the definition.
(I've seen this before)
'cat biting a dog's tail' is the wordplay.
'cat' becomes 'mog' (I've seen this before**).
'biting' indicates putting letters inside (I've seen this in other clues).
'tail' says to take the final letters.
The last letter of 'dog' is 'g'.
'a'+'g'='ag'
'mog' going around 'ag' is 'MAGOG'.
(Other definitions for magog that I've seen before include "Gog and -, City of London guardian giants" , "old giant" , "Biblical king" , "Gog's companion" , "Last of two old giants" .)