Is a sign of the Left extremity (7)
I believe the answer is:
portend
'is a sign' is the definition.
I can't tell whether this definition defines the answer.
'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' acts as a link.
Can you help me to learn more?
(Other definitions for portend that I've seen before include "Omen" , "forecast" , "Be a sign of something to come" , "Threaten" , "Give warning" .)