Nautical agreement where companion given role aboard vessel (7,5)
I believe the answer is:
charter party
'nautical agreement' is the definition.
(maritime agreement or contract)
'companion given role aboard vessel' is the wordplay.
'companion' becomes 'ch' (Companion of Honour).
'given' says to put letters next to each other.
'role' becomes 'part' (eg a part or role in a film).
'aboard' is an insertion indicator.
'vessel' becomes 'artery' (artery is a kind of vessel).
'part' placed inside 'artery' is 'arterparty'.
'ch'+'arterparty'='CHARTER-PARTY'
'where' acts as a link.
(Another definition for charter party that I've seen is " Deed".)