Charge, on average, is reasonable (8)
I believe the answer is:
moderate
'reasonable' is the definition.
(similar in meaning)
'charge on average' is the wordplay.
'charge' becomes 'rate' (rate is a kind of charge).
'on' means one lot of letters go next to another.
'average' becomes 'mode' (mode is a kind of average).
'rate' after 'mode' is 'MODERATE'.
'is' acts as a link.
(Other definitions for moderate that I've seen before include "Chair" , "Make reasonable" , "Restrained" , "Mild" , "not going to extremes" .)