Watch old boy wait (7)
I believe the answer is:
observe
'watch' is the definition.
(observing is a kind of watching)
'old boy wait' is the wordplay.
'old boy' becomes 'ob' (abbreviation).
'wait' becomes 'serve' (I've seen this before).
'ob'+'serve'='OBSERVE'
(Other definitions for observe that I've seen before include "Notice or make a remark" , "Notice, perceive" , "monitor" , "Obverse (anag.)" , "Watch without intervening" .)