Remarkable theory wont break down (10)
I believe the answer is:
noteworthy
'remarkable' is the definition.
('noteworthy' can be a synonym of 'remarkable')
'theory wont break down' is the wordplay.
'break down' indicates an anagram.
'theory'+'wont'='theorywont'
'theorywont' is an anagram of 'NOTEWORTHY'.
(Other definitions for noteworthy that I've seen before include "important" , "Interesting, significant" , "Signal" , "Of significance" .)