Keen-eyed alumnus with man (9)
I believe the answer is:
observant
'keen-eyed' is the definition.
(I've seen this before)
'alumnus with man' is the wordplay.
'alumnus' becomes 'ob' (old boy).
'with' is a charade indicator (letters next to each other).
'man' becomes 'servant' (I've seen this before).
'ob'+'servant'='OBSERVANT'
(Other definitions for observant that I've seen before include "keeping close eye on things" , "Alert" , "following religious practices" , "Watchful" , "Orthodox" .)