Roc, this fantastic bird (7)
I believe the answer is:
ostrich
'bird' is the definition.
(I know that ostrich is a type of flightless bird)
'roc this fantastic' is the wordplay.
'fantastic' is an anagram indicator (a fantastic or improbable ordering of the letters).
'roc'+'this'='rocthis'
'rocthis' with letters rearranged gives 'OSTRICH'.
(Other definitions for ostrich that I've seen before include "Rich sot seeks large bird" , "Large flightless bird with fancy plumes" , "Emu-like bird" , "Largest extant bird" , "World's largest bird" .)