Signal left on board and stop (7)
I believe the answer is:
portend
'signal' is the definition.
(portending is a kind of signalling)
'left on board and stop' is the wordplay.
'left on board' becomes 'port' (port is left on board a ship).
'and' is a charade indicator (letters next to each other).
'stop' becomes 'end' ('end' can be a synonym of 'stop').
'port'+'end'='PORTEND'
(Other definitions for portend that I've seen before include "Foreshadow" , "Be a sign of (impending calamity)" , "Give warning" , "Be a sign of something to come" , "forecast" .)