If (7)

I think it could be either:

Which length description are you looking for?:

whether

'if' is the definition.
(synonyms)

This is all the clue.

(Other definitions for whether that I've seen before include "- - 'tis nobler in the mind... Shakespeare's 'Hamlet'" , "Word introduces alternative" , "He threw (anag.)" , "Whichever is the case" , "-- or no means in any event" .)

I've seen this clue in The Telegraph and The Guardian.

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