Predator of rat, say, biting its head off and beginning to eat (6)
I believe the answer is:
ermine
'predator' is the definition.
(ermines are predators)
'rat say biting its head off and beginning to eat' is the wordplay.
'rat say' becomes 'vermin' (rats are vermin).
'biting its head off' means to remove the first letter.
'and' says to put letters next to each other.
'beginning to' says to take the initial letters.
The initial letter of 'eat' is 'e'.
'vermin' with its initial letter taken off is 'ermin'.
'ermin'+'e'='ERMINE'
'of' is the link.
(Other definitions for ermine that I've seen before include "White fur worn by some judges" , "White fur of weasel" , "special trimmings" , "Ceremonial fur" , "Expensive white fur" .)