Slightly over medium range (5)
I believe the answer is:
ambit
'range' is the definition.
(I know that range can be written as ambit)
'slightly over medium' is the wordplay.
'slightly' becomes 'a bit' ('a bit' can mean 'slightly' or 'somewhat').
'over' means one lot of letters goes inside another.
'medium' becomes 'm' (as in UK clothing sizes).
'abit' placed around 'm' is 'AMBIT'.
(Other definitions for ambit that I've seen before include "Scope or confines" , "Scope, extent, or bounds of something" , "round" , "Compass, scope" , "Scope or circuit" .)