Regret hosting thousands on king's vessel (6)
I believe the answer is:
rummer
'vessel' is the definition.
(type of drinking vessel)
'regret hosting thousands on king's' is the wordplay.
'regret' becomes 'rue' (to rue something is to regret it).
'hosting' means one lot of letters goes inside another (inserted letters are welcomed in or hosted).
'thousands' becomes 'mm' (Roman numerals).
'on' means one lot of letters go next to another.
'king' becomes 'R' (abbreviation of Latin rex).
'rue' going around 'mm' is 'rumme'.
'rumme'+'r'='RUMMER'
(Other definitions for rummer that I've seen before include "Comparatively quaint" , "glass in the bar" , "Stranger" , "Stemmed drinking glass" , "drinking-vessel" .)