Hang on, beginning to get aboard vessel (6)
I believe the answer is:
linger
'hang' is the definition.
(thesaurus)
'beginning to get aboard vessel' is the wordplay.
'beginning to' says to take the initial letters.
'aboard' indicates putting letters inside.
'vessel' becomes 'liner' (liner is a kind of vessel**).
The first letter of 'get' is 'g'.
'g' going inside 'liner' is 'LINGER'.
'on' acts as a link.
(Other definitions for linger that I've seen before include "Dawdle, hang around" , "Hang back awhile" , "Be slow to leave" , "Procrastinate" , "Hang about, don't be in a hurry to go" .)