Whenever (2,6)

I think it could be either:

Which length description are you looking for?:

2,6

3,4

on demand

'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" .)

I've seen this clue in the Newsday.

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