Sensible friend crossing Oder (8)
I believe the answer is:
moderate
'sensible' is the definition.
(thesaurus)
'friend crossing oder' is the wordplay.
'friend' becomes 'mate' (mate is a kind of friend).
'crossing' means one lot of letters goes inside another (to cross can mean to go over or around).
'mate' enclosing 'oder' is 'MODERATE'.
(Other definitions for moderate that I've seen before include "So-so; sensible" , "Average in amount" , "Slacken" , "Average; not radical" , "Unextreme" .)