Brace of flightless birds, fast but not hot (8)
I believe the answer is:
reassure
'brace' is the definition.
(thesaurus)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for reassure that I've seen before include "Take away a person's doubts or fears" , "Comfort, convince again" , "Give confidence to" , "Buoy up" , "give encouragement" .)