He's very keen to have a cash surplus (5)
I believe the answer is:
lover
'he's very keen to' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'a cash surplus' is the wordplay.
'a cash' becomes 'l' (The pound sterling symbol is an elaborate L).
'surplus' becomes 'over' (I've seen this in other clues).
'l'+'over'='LOVER'
'have' acts as a link.
Can you help me to learn more?
(Other definitions for lover that I've seen before include "Beau" , "He really likes" , "Suitor" , "fan" , "admirer" .)