Dutiful retainer following old boy (9)
I believe the answer is:
observant
'dutiful' is the definition.
(thesaurus)
'retainer following old boy' is the wordplay.
'retainer' becomes 'servant' ('servant' can be a synonym of 'retainer').
'following' is a charade indicator (letters next to each other).
'old boy' becomes 'ob'.
'servant' put after 'ob' is 'OBSERVANT'.
(Other definitions for observant that I've seen before include "paying close attention" , "Keen-eyed" , "Sharp-eyed" , "(Of music)using conventional keys" , "attentive to detail" .)