Warn of downfall supporting Left (7)
I believe the answer is:
portend
'warn' is the definition.
(thesaurus)
'downfall supporting left' is the wordplay.
'downfall' becomes 'end' (I've seen this in another clue).
'supporting' says to put letters next to each other (in a down clue, some letters go below others).
'left' becomes 'port' (left side of a ship - opposite of starboard).
'end' after 'port' is 'PORTEND'.
'of' is the link.
(Other definitions for portend that I've seen before include "Foreshadow" , "Give warning" , "forecast" , "Betoken; presage" , "Presage, bode" .)