Former pupil, retainer, watchful (9)
I believe the answer is:
observant
'watchful' is the definition.
(thesaurus)
'former pupil retainer' is the wordplay.
'former pupil' becomes 'ob' (abbreviation for 'Old Boy').
'retainer' becomes 'servant' ('servant' can be a synonym of 'retainer').
'ob'+'servant'='OBSERVANT'
(Other definitions for observant that I've seen before include "Likely to notice" , "(Of music)using conventional keys" , "Perceptive" , "Quick at noticing things, attentive" , "paying close attention" .)