Only trace found by old boy is out of date (11)
I believe the answer is:
obsolescent
'out of date' is the definition.
(thesaurus)
'only trace found by old boy' is the wordplay.
'only' becomes 'sole' (synonyms).
'trace' becomes 'scent' (I've seen this in another clue).
'found by' says to put letters next to each other.
'old boy' becomes 'ob' (abbreviation).
'sole'+'scent'='solescent'
'solescent' after 'ob' is 'OBSOLESCENT'.
'is' is the link.
(Other definitions for obsolescent that I've seen before include "out of fashion" , "Getting dated" , "past its sell-by date?" , "losing currency" , "almost out of date" .)