Big bird to be unsteady, mostly (3)
I believe the answer is:
roc
'big bird' is the definition.
(mythical giant bird)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for roc that I've seen before include "Large bird of legend" , "Sinbad's giant bird" , "Vast legendary bird" , "Fabulous giant bird of Eastern legend" , "Sinbad's monster bird" .)