What's said to be average appearance (4)
I believe the answer is:
mien
'appearance' is the definition.
(I've seen this in another clue)
'what's said to be average' is the wordplay.
'what's said to be' shows a homophone (sound like).
'average' becomes 'mean' (type of average).
'mean' is a homophone of 'MIEN'.
(Other definitions for mien that I've seen before include "Appearance, bearing" , "A face like mine" , "Dignified manner" , "Bearing, demeanour" , "attitude" .)