Beetle brings endless alarm to sailor (6)
I believe the answer is:
scarab
'beetle' is the definition.
(scarab is a kind of beetle)
'endless alarm to sailor' is the wordplay.
'endless' means to remove the last letter.
'alarm' becomes 'scare' (I've seen this before).
'to sailor' becomes 'ab' (abbreviation for able seaman**).
'scare' with its last letter taken away is 'scar'.
'scar'+'ab'='SCARAB'
'brings' acts as a link.
(Other definitions for scarab that I've seen before include "Sacred beetle of ancient Egypt" , "Large dark beetle" , "insect" , "Beetle, sacred to the ancient Egyptians" , "Sacred dung-beetle" .)