Nothing special (4)
'nothing special' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for average that I've seen before include "Mediocre - medium" , "Mean - norm" , "Normal in arithmetic way" , "About normal or usual" , "Mediocre, commonplace" .)
(Other definitions for soso that I've seen before include "Indifferent" , "mediocre" .)