Warn of the left extremity (7)
I believe the answer is:
portend
'warn' is the definition.
(thesaurus)
'left extremity' is the wordplay.
'left' becomes 'port' (left side of a ship - opposite of starboard).
'extremity' becomes 'end' (end is a kind of extremity).
'port'+'end'='PORTEND'
'of the' is the link.
(Other definitions for portend that I've seen before include "Signal" , "Foreshadow" , "Be a sign of (impending calamity)" , "Presage, bode" , "Give warning" .)