This roc, monstrous bird (7)
I believe the answer is:
ostrich
'bird' is the definition.
(ostrich is a kind of bird)
'this roc monstrous' is the wordplay.
'monstrous' indicates anagramming the letters.
'this'+'roc'='thisroc'
'thisroc' is an anagram of 'OSTRICH'.
(Other definitions for ostrich that I've seen before include "One refusing to face facts; flightless African bird" , "Bird burying its head in the sand" , "A bird for the shirt co." , "Creature" , "Largest extant bird" .)