Old boy and lackey alert (9)
I believe the answer is:
observant
'alert' is the definition.
(alert can mean observant or vigilant)
'old boy and lackey' is the wordplay.
'old boy' becomes 'OB' (abbreviation for a private school 'old boy').
'and' means one lot of letters go next to another.
'lackey' becomes 'servant' (I've seen this before).
'ob'+'servant'='OBSERVANT'
(Other definitions for observant that I've seen before include "attentive to detail" , "Keen-eyed" , "with a keen eye" , "keeping close eye on things" , "Sharp-eyed" .)