Keen-eyed alumnus with lackey (9)
I believe the answer is:
observant
'keen-eyed' is the definition.
(I've seen this before)
'alumnus with lackey' is the wordplay.
'alumnus' becomes 'ob' (old boy, as in an old private school pupil).
'with' means one lot of letters go next to another.
'lackey' becomes 'servant' (I've seen this in another clue).
'ob'+'servant'='OBSERVANT'
(Other definitions for observant that I've seen before include "paying close attention" , "Heedful" , "diligent in duty" , "with a keen eye" , "Good at noticing" .)