Like helium (5)

I think it could be either:

Which length description are you looking for?:

inert

'like helium' is the definition.
(I've seen this in another clue)

This is all the clue.

(Other definitions for inert that I've seen before include "Nothing doing" , "Motionless or inanimate" , "Lifeless, without power to move or act" , "Not active or reactive" , "dead" .)

I've seen this clue in The New York Times and the LA Times.

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