Good old boy allowed to see Grail? (6)
I believe the answer is:
goblet
'grail?' is the definition.
(grail is a kind of goblet)
'good old boy allowed to' is the wordplay.
'good' becomes 'g' (abbreviation).
'old boy' becomes 'OB' (abbreviation for a private school 'old boy').
'allowed to' becomes 'let' (to let something happen is to allow it).
'g'+'ob'+'let'='GOBLET'
'see' is the link.
(Other definitions for goblet that I've seen before include "Drinking-glass with stem and foot" , "Large drinking cup" , "Drinking glass with stem and foot" , "Chalice" , "Large drinking glass with stem and base" .)