On average, is capital (5)
I believe the answer is:
paris
'capital' is the definition.
(Paris is the capital of France)
'on average is' is the wordplay.
'on' is a charade indicator (letters next to each other).
'average' becomes 'par' (I've seen this before).
'par'+'is' is 'PARIS'.
(Other definitions for paris that I've seen before include "Arc de Triomphe city" , "European capital city" , "Helen's abductor" , "English chronicler" , "where he got plastered?" .)