Queen aboard pretentious vessel (6)
I believe the answer is:
artery
'vessel' is the definition.
(artery is a kind of vessel)
'queen aboard pretentious' is the wordplay.
'queen' becomes 'er' (abbreviation for Elizabeth Regina).
'aboard' means one lot of letters goes inside another.
'pretentious' becomes 'arty' ('arty' can be similar in meaning to 'pretentious').
'er' going inside 'arty' is 'ARTERY'.
(Other definitions for artery that I've seen before include "main line" , "vessel carrying vital supply" , "Something that could be clogged" , "Major blood vessel or road" , "Vital tube" .)