Stadium slashed flat charge (6,4)
I believe the answer is:
ground rent
'flat charge' is the definition.
(type of rent payment)
'stadium slashed' is the wordplay.
'stadium' becomes 'ground' (eg a football ground).
'slashed' becomes 'rent' (I've seen this before).
'ground'+'rent'='GROUND-RENT'
(Other definitions for ground rent that I've seen before include "Payment for lease of land" , "Result of earth-shattering" , "Payment to landowner" .)