Watchful attendant behind old boy (9)
I believe the answer is:
observant
'watchful' is the definition.
(thesaurus)
'attendant behind old boy' is the wordplay.
'attendant' becomes 'servant' (I've seen this before).
'behind' means one lot of letters go next to another.
'old boy' becomes 'ob'.
'servant' after 'ob' is 'OBSERVANT'.
(Other definitions for observant that I've seen before include "keeping close eye on things" , "paying close attention" , "Quick at noticing things, attentive" , "Likely to notice" , "adhering to religious practices" .)