Medium-size, unopened concoction for evacuation (6)
I believe the answer is:
motion
'evacuation' is the definition.
The answer and definition can be both acts as well as being singular nouns.
Perhaps they are linked in a way I don't understand?
'medium-size unopened concoction' is the wordplay.
'medium size' becomes 'm' (I can't explain this - if you can you should give a lot more credence to this answer).
'unopened' means to remove the first letter (without opening letter).
'concoction' becomes 'potion' (I've seen this in another clue).
'potion' with its initial letter taken away is 'otion'.
'm'+'otion'='MOTION'
'for' is the link.
Can you help me to learn more?
(Other definitions for motion that I've seen before include "Formal proposal put to a meeting" , "Proposal for discussion or a vote" , "That's debatable" , "Movement or proposal" , "Proposal to be voted on" .)