Promise onboard weapons (6)
I believe the answer is:
swords
'weapons' is the definition.
(sword is a kind of weapon)
'promise onboard' is the wordplay.
I cannot quite understand how this works, but
'promise' could be 'word' (as in 'I give you my word') and 'word' is located in the answer.
This explanation may well be incorrect...
(Other definitions for swords that I've seen before include "Steel weapons" , "Weapons from north Co Dublin?" , "Eg, rapiers, sabres" , "Fighting blades" , "Weapons; a Tarot suit" .)