Say to the old boy 'Wait' (7)
I believe the answer is:
observe
'say' is the definition.
(observing is a kind of saying)
'the old boy wait' is the wordplay.
'the old boy' becomes 'ob'.
'wait' becomes 'serve' (I've seen this before).
'ob'+'serve'='OBSERVE'
'to' acts as a link.
(Other definitions for observe that I've seen before include "Obverse (anag.)" , "Watch, take note of" , "Scrutinise or comply with" , "Look at" , "Espy" .)