Bomb danger at sea ultimately preventable (7)
I believe the answer is:
grenade
'bomb' is the definition.
(grenade is a kind of bomb)
'danger at sea ultimately preventable' is the wordplay.
'at sea' indicates an anagram.
'ultimately' suggests the final letters.
The last letter of 'preventable' is 'e'.
'danger' with letters rearranged gives 'grenad'.
'grenad'+'e'='GRENADE'
(Other definitions for grenade that I've seen before include "Small explosive device" , "Bomb with a pin" , "Handy bomb" , "ultimate weapon" , "Small hand-bomb" .)