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 there's a link between them I don't understand?
'medium-size unopened concoction' is the wordplay.
'medium size' becomes 'm' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'unopened' means to remove the first letter (the opening of the word is removed).
'concoction' becomes 'potion' (I've seen this before).
'potion' with its first letter removed is 'otion'.
'm'+'otion'='MOTION'
'for' acts as a 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" .)