Intended that man to support left-winger in plot (9)
I believe the answer is:
betrothed
'intended' is the definition.
(I've seen this before)
'man to support left-winger in plot' is the wordplay.
'man' becomes 'he' (he refers to a man).
'to support' is a charade indicator (letters next to each other) (in a down clue, some letters hold up others).
'left-winger' becomes 'trot' (short for Trotskyite).
'in' is an insertion indicator.
'plot' becomes 'bed' (a plot of land).
'he' after 'trot' is 'trothe'.
'trothe' inserted inside 'bed' is 'BETROTHED'.
'that' is the link.
(Other definitions for betrothed that I've seen before include "committed somehow" , "Engaged - fiance" , "Spoken for" , "Engaged or affianced" , "Promised in marriage" .)