Outstanding old boy with unusual virtues (9)
I believe the answer is:
obtrusive
'outstanding' is the definition.
(thesaurus)
'old boy with unusual virtues' is the wordplay.
'old boy' becomes 'OB' (abbreviation for a private school 'old boy').
'with' is a charade indicator (letters next to each other).
'unusual' indicates an anagram.
'virtues' is an anagram of 'trusive'.
'ob'+'trusive'='OBTRUSIVE'
(Other definitions for obtrusive that I've seen before include "Unduly prominent" , "Unwelcomely prominent" , "Unpleasantly noticeable or prominent" , "Noticeably prominent" , "Over-prominent" .)