Forewarn of harbour to close (7)
I believe the answer is:
portend
'forewarn' is the definition.
(thesaurus)
'harbour to close' is the wordplay.
'harbour' becomes 'port' (I've seen this before).
'to close' becomes 'end' (I've seen this before).
'port'+'end'='PORTEND'
'of' is the link.
(Other definitions for portend that I've seen before include "Foreshadow" , "Signal" , "Omen" , "Betoken; presage" , "forecast" .)