Unexceptional, thus repeated (2-2)
I believe the answer is:
so-so
'unexceptional' is the definition.
'thus repeated' is the wordplay.
'thus' becomes 'so' (synonyms).
'repeated' shows that letters should be duplicated.
'so' doubled is 'SO-SO'.
(Other definitions for so-so that I've seen before include "Of middling quality" , "Barely passable" , "'Passable, middling (2-2)'" , "Neither good nor bad" , "Fair to middling" .)