Tom's wrong about Des being unassuming (6)
I believe the answer is:
modest
'unassuming' is the definition.
(similar in meaning)
'tom's wrong about des' is the wordplay.
'wrong' indicates anagramming the letters.
'about' indicates putting letters inside.
'tom' is an anagram of 'mot'.
'mot' going around 'des' is 'MODEST'.
'being' acts as a link.
(Other definitions for modest that I've seen before include "Unassuming or unprententious" , "Inexpensive" , "Unassuming or decent" , "Bashful" , "'Unassuming, diffident (6)'" .)