Old boy going to do to celebrate (7)
I believe the answer is:
observe
'celebrate' is the definition.
(I've seen this before)
'old boy going to do' is the wordplay.
'old boy' becomes 'ob'.
'going to' says to put letters next to each other.
'do' becomes 'serve' (serving is a kind of doing).
'ob'+'serve'='OBSERVE'
'to' is the link.
(Other definitions for observe that I've seen before include "Watch closely" , "Verbose (anag.)" , "Spot, watch" , "Pay attention" , "Watch, take note of" .)