Old boy to function as witness (7)
I believe the answer is:
observe
'witness' is the definition.
(I've seen this in another clue)
'old boy to function' is the wordplay.
'old boy' becomes 'OB' (abbreviation for a private school 'old boy').
'to function' becomes 'serve' (serving is a kind of functioning).
'ob'+'serve'='OBSERVE'
'as' acts as a link.
(Other definitions for observe that I've seen before include "Keep; notice" , "Discern" , "Notice; study" , "Espy" , "Witness, watch" .)