Freely (2,4)
I believe the answer is:
at will
'freely' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for at will that I've seen before include "Fire .. .... - whenever you're ready" , "whenever you want" , "Whenever one pleases, own choice" , "Freely" , "Whenever one wishes" .)