Runner accepts cost for getting treatment — what needs paying? (7,3)

'what needs paying?' is the definition.
(payment to a doctor)

'runner accepts cost for getting treatment' is the wordplay.
'runner' becomes 'dee' (River Dee).
'accepts' means one lot of letters goes inside another.
'getting treatment' indicates an anagram.
'cost'+'for'='costfor'
'costfor' with letters rearranged gives 'octorsf'.
'dee' enclosing 'octorsf' is 'DOCTORS FEE'.