As, say, many momentous money matters — trial, leave it to be sorted out? (12)
I believe the answer is:
alliterative
'as say many momentous money matters' is the definition.
('many momentous money matters' is alliterative)
'trial leave it to be sorted out?' is the wordplay.
'to be sorted out?' indicates an anagram.
'trial'+'leave'+'it'='trialleaveit'
'trialleaveit' with letters rearranged gives 'ALLITERATIVE'.
(Other definitions for alliterative that I've seen before include "Starting with identical letters" , "Starting with the same letter or sound" .)