Unostentatious (6)
I believe the answer is:
modest
'unostentatious' is the definition.
(synonyms)
This is all the clue.
(Other definitions for modest that I've seen before include "'Unassuming, diffident (6)'" , "Demure" , "Not boastful" , "Lacking vanity" , "Not conceited, unassuming" .)