Casting off, huge breaks they can't believe (9)
I believe the answer is:
agnostics
'they can't believe' is the definition.
The definition suggests an adverb but the answer is not.
'casting off huge breaks' is the wordplay.
'off' is an anagram indicator.
'huge' becomes 'os' (abbreviation for outsize).
'breaks' means one lot of letters goes inside another (inserted letters break into the word).
'casting' is an anagram of 'agntics'.
'agntics' going around 'os' is 'AGNOSTICS'.
Can you help me to learn more?
(Other definitions for agnostics that I've seen before include "They refuse to believe" , "They claim one can't have true knowledge of the existence of God" , "They don't know if God exists or not" , "Sceptics" .)