Men committed to union securing new funds (8)
I believe the answer is:
finances
'funds' is the definition.
(synonyms)
'men committed to union securing new' is the wordplay.
'men committed to union' becomes 'fiances' (committed to marriage).
'securing' is an insertion indicator.
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'fiances' placed around 'n' is 'FINANCES'.
(Other definitions for finances that I've seen before include "Provides the capital" , "Monetary resources" , "funds" , "Money resources as of country or company" , "Assets in the form of money" .)