Demands that one does it at once (6)
I believe the answer is:
exacts
'demands' is the definition.
(I know this)
'one does it at once' is the wordplay.
'one' becomes 'a' (a thing is one thing).
'does it' becomes 'cts' (I can't justify this - if you can you should give a lot more credence to this answer).
'at' is a charade indicator (letters next to each other) (I've seen this in other clues).
'once' becomes 'ex' (prefix meaning former or onetime).
'a'+'cts'='acts'
'acts' after 'ex' is 'EXACTS'.
'that' acts as a link.
Can you help me to learn more?
(Other definitions for exacts that I've seen before include "Insists on or claims" , "making demands" , "Extorts" , "Insists on and obtains - sound precise" , "Insists on and obtains, say payment - sounds precise" .)