Socialist shortly gets up and leaves (3)
I believe the answer is:
cos
'leaves' is the definition.
(cos lettuce leaves)
'socialist shortly gets up' is the wordplay.
'shortly' indicates taking the first letters.
'gets' becomes 'oc' (I can't explain this - if you can you should believe this answer much more).
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
The first letter of 'socialist' is 's'.
's'+'oc'='soc'
'soc' reversed gives 'COS'.
'and' is the link.
Can you help me to learn more?
(Other definitions for cos that I've seen before include "Mozart fidelity-test opera" , "Kind of lettuce in microcosm" , "Greek island (alternative spelling): Trig abbrev!" , "Greek island and type of lettuce" , "something of a salad" .)