Demands that one does, at once (6)
I believe the answer is:
exacts
'demands' is the definition.
('exact' can be a synonym of 'demand')
'one does at once' is the wordplay.
'one does' becomes 'acts' (I've seen this before).
'at' means one lot of letters go next to another (I've seen this in other clues).
'once' becomes 'ex' (prefix meaning former or onetime).
'acts' after 'ex' is 'EXACTS'.
'that' is the 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" .)