Trouble ahead over harbours (3)
I believe the answer is:
ado
'trouble' is the definition.
(thesaurus)
'ahead over harbours' is the wordplay.
'harbours' says the answer is hidden in the clue.
'ADO' is hidden in the letters of 'ahead over'.
(Other definitions for ado that I've seen before include "Song and dance" , "agitation" , "'Much ... About Nothing, Shakespeare play (3)'" , "Fuss or bustle" , "Busy activity, trouble" .)