Live with a rook: ferocious creature! (4)
I believe the answer is:
bear
'ferocious creature' is the definition.
'bear' can be an answer for 'creature' (bear is a kind of creature). I'm not certain of the 'ferocious' bit.
'live with a rook' is the wordplay.
'live' becomes 'be' (synonyms).
'with' says to put letters next to each other.
'rook' becomes 'R' (chess abbreviation).
'be'+'a'+'r'='BEAR'
Can you help me to learn more?
(Other definitions for bear that I've seen before include "Carry; large mammal" , "Support; produce (fruit)" , "a churlish individual" , "rude fellow" , "Massive mammal" .)