Sustain the ruined financier (10)
I believe the answer is:
enthusiast
'financier' is the definition.
The definition and answer can be both people as well as being singular nouns.
Perhaps you can see an association between them that I can't see?
'sustain the ruined' is the wordplay.
'ruined' is an anagram indicator.
'sustain'+'the'='sustainthe'
'sustainthe' anagrammed gives 'ENTHUSIAST'.
Can you help me to learn more?
(Other definitions for enthusiast that I've seen before include "buff" , "one not at all cool" , "Devotee" , "Fan" , "Ardent supporter" .)