Bird is engrossed over rook (6)
I believe the answer is:
raptor
'bird' is the definition.
(raptor is a kind of bird)
'engrossed over rook' is the wordplay.
'engrossed' becomes 'rapt' (synonyms).
'over' becomes 'o' (cricket abbreviation).
'rook' becomes 'R' (chess abbreviation).
'rapt'+'o'+'r'='RAPTOR'
'is' acts as a link.
(Other definitions for raptor that I've seen before include "Harrier possibly" , "Hunter" , "Eagle, hawk, falcon or owl - parrot (anag)" , "Parrot (anag) - bird of prey" .)