Old boy abandoning watchful attendant (7)
I believe the answer is:
servant
'attendant' is the definition.
(I've seen this before)
'old boy abandoning watchful' is the wordplay.
'old boy' becomes 'OB' (abbreviation for a private school 'old boy').
'abandoning' suggests deleting specific letters.
'watchful' becomes 'observant' (I've seen this before).
'observant' with 'ob' taken away is 'SERVANT'.
(Other definitions for servant that I've seen before include "Page, say" , "One working for another" , "Valet, e.g." , "Perhaps chauffeur" , "Vest ran (anag)" .)