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.
Maybe they are linked in a way I don't understand?
'medium-size unopened concoction' is the wordplay.
'medium size' becomes 'm' (I can't justify 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 before).
'potion' with its first letter taken off 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" .)