Notice for travellers at sea: boarding's not starting (4,4)
I believe the answer is:
road sign
'notice for travellers' is the definition.
'at sea boarding's not starting' is the wordplay.
'at sea' indicates an anagram.
'not starting' means to remove the first letter.
'boardings' with its first letter taken away is 'oardings'.
'oardings' is an anagram of 'ROAD SIGN'.
(Other definitions for road sign that I've seen before include "Roundabout ahead or T-junction, say" , "something that can be seen along the way" , "Bearer of warning" , "Stop or Yield, say" , "Info for drivers" .)