Sharp former pupil gets domestic (9)
I believe the answer is:
observant
'sharp' is the definition.
(thesaurus)
'former pupil gets domestic' is the wordplay.
'former pupil' becomes 'OB' (abbreviation for 'Old Boy' - private school pupil).
'gets' means one lot of letters go next to another.
'domestic' becomes 'servant' (I've seen this before).
'ob'+'servant'='OBSERVANT'
(Other definitions for observant that I've seen before include "Orthodox" , "Quick to notice things" , "Watchful" , "(Of music)using conventional keys" , "taking things in" .)