Whenever (3,4)
(Other definitions for any time that I've seen before include "at one's disposal night and day" , "Whenever you like" .)
'whenever' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for on demand that I've seen before include "As soon as required" , "As soon as requested" , "When asked for" , "Whenever required" .)