Left supports socialist's leader jesting (5)
I believe the answer is:
'jesting' is the definition.
(sport can mean jesting or joking)
'left supports socialist's leader' is the wordplay.
'left' becomes 'port' (left side of a ship - opposite of starboard).
'supports' means one lot of letters go next to another (bottom letters provide a support for the top ones).
'socialist's leader' becomes 's' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'port' put after 's' is 'SPORT'.
Can you help me to learn more?
(Other definitions for sport that I've seen before include "Fun; wear" , "Put on" , "Games" , "Recreation, play" , "Wear, display (an item)" .)