Weapons showing promise on board ship (6)
I believe the answer is:
swords
'weapons' is the definition.
(sword is a kind of weapon)
'promise on board ship' is the wordplay.
'promise' becomes 'word' (as in 'I give you my word').
'on board' is an insertion indicator.
'ship' becomes 'ss'.
'word' put within 'ss' is 'SWORDS'.
'showing' is the link.
(Other definitions for swords that I've seen before include "Steel weapons" , "Eg, rapiers, sabres" , "Weapons from north Co Dublin?" , "Bladed weapons in north Dublin" , "Weaponry" .)