Title of private vessel the Queen goes aboard (9)
I believe the answer is:
ownership
'title' is the definition.
(legal term for ownership)
'private vessel the queen goes aboard' is the wordplay.
'private' becomes 'own' (I've seen this before).
'vessel' becomes 'ship' (ship is a kind of vessel**).
'the queen' becomes 'er' (Elizabeth Regina).
'goes aboard' is an insertion indicator.
'own'+'ship'='ownship'
'ownship' going around 'er' is 'OWNERSHIP'.
'of' acts as a link.
(Other definitions for ownership that I've seen before include "Proprietorship, possession" , "Legal title" , "The right of possessing something" , "Proprietary right of possession" .)