Appear as left-winger with following of not quite 50 per cent engagement (9)
I believe the answer is:
betrothal
'engagement' is the definition.
(I know that engagement can be written as betrothal)
'appear as left' is the wordplay.
'appear as' becomes 'betroth' (I can't explain this - if you can you should believe this answer much more).
'left' becomes 'al' (I've seen this in other clues).
'betroth'+'al'='BETROTHAL'
'winger with following of not quite 50 per cent' is the link.
This may not be right. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for betrothal that I've seen before include "Commitment" , "Pledge" , "Promise between two people" , "Engagement to marry" , "Plight" .)