Doctor saves nothing on average (8)
I believe the answer is:
mediocre
'average' is the definition.
(I know that average can be written as mediocre)
'doctor saves nothing on' is the wordplay.
'doctor' becomes 'medic' (doctor is a type of medic).
'saves' is an insertion indicator.
'nothing' becomes 'O' (looks like zero - 0).
'on' becomes 're' ('on' can be mean 'with reference to').
'medic' enclosing 'o' is 'medioc'.
'medioc'+'re'='MEDIOCRE'
(Other definitions for mediocre that I've seen before include "Commonplace, not distinguished" , "Come ride (anag.)" , "poor to middling" , "Fair, middle of the road" , "Ordinary" .)