Demands that one does, at once (6)
I believe the answer is:
exacts
'demands' is the definition.
(exacting is a kind of demanding)
'one does at once' is the wordplay.
'one does' becomes 'acts' (I've seen this before).
'at' says to put letters next to each other (I've seen this in other clues).
'once' becomes 'ex'.
'acts' after 'ex' is 'EXACTS'.
'that' acts as a link.
(Other definitions for exacts that I've seen before include "Insists on and obtains - sound precise" , "making demands" , "Extorts" , "Compels payment of" , "Insists on and obtains, say payment - sounds precise" .)