Alumnus to function as witness (7)
I believe the answer is:
observe
'witness' is the definition.
(I've seen this before)
'alumnus to function' is the wordplay.
'alumnus' becomes 'ob' (old boy, as in an old private school pupil).
'to function' becomes 'serve' (serving is a kind of functioning).
'ob'+'serve'='OBSERVE'
'as' is the link.
(Other definitions for observe that I've seen before include "See - say" , "Spot, watch" , "Keep an eye on" , "See - keep - remark - maintain - celebrate" , "Scrutinise or comply with" .)