Boarding vessel, choose the best oars (6)
I believe the answer is:
sculls
'oars' is the definition.
(scull is a kind of oar)
'boarding vessel choose the best' is the wordplay.
'boarding' indicates putting letters inside.
'vessel' becomes 'SS' (prefix in ship names eg SS Great Britain).
'choose the best' becomes 'cull' (I've seen this in another clue).
'ss' placed around 'cull' is 'SCULLS'.
(Other definitions for sculls that I've seen before include "Small oars used by single rowers" , "Oars; race" , "Pair of short oars used by single oarsman" , "Pair of small oars" , "Small oars used by single rower" .)