Fish in boat with attractive weapon (9)
I believe the answer is:
slingshot
'weapon' is the definition.
(I've seen this before)
'fish in boat with attractive' is the wordplay.
'fish' becomes 'ling' (ling is a kind of fish).
'in' is an insertion indicator.
'boat' becomes 'SS' (prefix in ship names eg SS Great Britain).
'with' means one lot of letters go next to another.
'attractive' becomes 'hot' (I've seen this in another clue).
'ling' put inside 'ss' is 'slings'.
'slings'+'hot'='SLINGSHOT'
(Other definitions for slingshot that I've seen before include "fatal to Goliath" , "No slights (anag) - catapult (US)" , "that can help you get a stone off" , "enough to knock you out" , "Weapon American kid may produce" .)