Besides having a greater surplus (8)
I believe the answer is:
moreover
'besides' is the definition.
('moreover' can be a synonym of 'besides')
'a greater surplus' is the wordplay.
'a greater' becomes 'more'.
'surplus' becomes 'over' (I've seen this before).
'more'+'over'='MOREOVER'
'having' is the link.
(Other definitions for moreover that I've seen before include "Besides, in addition" , "as well" , "Furthermore, in addition" .)