Breathtakingly restrained (5)
I believe the answer is:
bated
'restrained' is the definition.
(bating is a kind of restraining)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for bated that I've seen before include "With - breath means in great suspense" , "Moderated, the breath in anticipation, say" , "Reduced (referring to breath)" .)