They’re beyond belief at hold-ups (8)
I believe the answer is:
atheists
'they're beyond belief' is the definition.
The definition suggests an adverb but the answer is not.
'at hold-ups' is the wordplay.
'hold-ups' becomes 'heists' (I've seen this in another clue).
'at'+'heists'='ATHEISTS'
Can you help me to learn more?
(Other definitions for atheists that I've seen before include "They believe there are no gods" , "ungodly crew" , "Heathens" , "They don't believe in a deity" , "People who believe there is no god" .)