Signs left on board sent off (8)
I believe the answer is:
portents
'signs' is the definition.
(portent is a kind of sign)
'left on board sent off' is the wordplay.
'left on board' becomes 'port' (port is left on board a ship).
'off' indicates anagramming the letters.
'sent' is an anagram of 'ents'.
'port'+'ents'='PORTENTS'
(Other definitions for portents that I've seen before include "Signs of things about to happen" , "Ominous signs" , "Omens" .)