Average, thus doubled (2-2)
I believe the answer is:
so-so
'average' is the definition.
(I've seen this before)
'thus doubled' is the wordplay.
'thus' becomes 'so' (synonyms).
'doubled' shows that letters should be duplicated.
'so' doubled is 'SO-SO'.
(Other definitions for so-so that I've seen before include "just about adequate" , "'Passable, middling (2-2)'" , "Native of Seville" , "Neither good nor bad" , "Of middling quality" .)