Bankroll fiancé with open-ended investment (7)
I believe the answer is:
finance
'bankroll' is the definition.
(I've seen this in another clue)
'fiance with open-ended investment' is the wordplay.
'with' is an insertion indicator.
'open-ended investment' becomes 'n' (I can't justify this - if you can you should give a lot more credence to this answer).
'fiance' enclosing 'n' is 'FINANCE'.
Can you help me to learn more?
(Other definitions for finance that I've seen before include "Supply funds" , "Supporting rhino" , "Monetary support" , "Money affairs" , "funding" .)