Fancy room aboard ship for weapons (8)
I believe the answer is:
sidearms
'weapons' is the definition.
(I've seen this before)
'fancy room aboard ship' is the wordplay.
'fancy' becomes 'idea' (I've seen this before).
'room' becomes 'rm' (abbreviation for room).
'aboard' means one lot of letters goes inside another.
'ship' becomes 'SS' (prefix in ship names eg SS Great Britain).
'idea'+'rm'='idearm'
'idearm' placed into 'ss' is 'SIDEARMS'.
'for' is the link.
(Other definitions for sidearms that I've seen before include "We could kill" , "weapons" .)