Is under average for capital (5)
I believe the answer is:
paris
'for capital' is the definition.
(Paris is the capital of France)
'is under average' is the wordplay.
'under' says to put letters next to each other (in a down clue, some letters go below others).
'average' becomes 'par' (average score in golf).
'is' after 'par' is 'PARIS'.
(Other definitions for paris that I've seen before include "European capital city" , "City on the Seine" , "I spar (anag)" , "Legendary prince" , "investment resulted from his action" .)