Mythical bird bigger, so gigantic ultimately (3)
I believe the answer is:
roc
'mythical bird' is the definition.
(mythical giant bird)
'bigger so gigantic ultimately' is the wordplay.
'ultimately' indicates one should take the final letters.
The last letters of 'bigger so gigantic' are 'ROC'.
(Other definitions for roc that I've seen before include "Gigantic bird of eastern legend - cor!" , "Vast bird of legend" , "Arabian Nights bird" , "Mythical bird; sounds like boulder" , "Fabulous Eastern bird of some ferocity" .)