Domestic support for alumnus on the look-out (9)
I believe the answer is:
observant
'on the look-out' is the definition.
'domestic support for alumnus' is the wordplay.
'domestic' becomes 'servant' (I've seen this before).
'support for' means one lot of letters go next to another.
'alumnus' becomes 'ob' (old boy).
'servant' put after 'ob' is 'OBSERVANT'.
(Other definitions for observant that I've seen before include "Closely watching" , "Alert" , "Heedful" , "diligent in duty" , "paying close attention" .)