Unexceptional (8)
(Other definitions for so-so that I've seen before include "OK, but nothing special" , "Barely passable" , "Nothing to write home about" , "just about adequate" , "Mediocre" .)
'unexceptional' is the definition.
(thesaurus)
This is the entire clue.
(Other definitions for passable that I've seen before include "okay?" , "Fair" , "examinations should be!" , "Fairly good but not outstanding" , "OK" .)
'unexceptional' is the definition.
(similar in meaning)
This is all the clue.
(Other definitions for ordinary that I've seen before include "Customary" , "Unexceptional, quite usual" , "old bicycle" , "Normal, mundane" , "Not worthy of noting in X-Files" .)
'unexceptional' 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 "About normal or usual" , "Mean; standard" , "Arithmetically ordinary?" , "Normal in arithmetic way" , "Mean - typical" .)