Vain attempt you keep hidden (5)

I believe the answer is:

empty

'vain' is the definition.
(thesaurus)

'attempt you keep hidden' is the wordplay.
'hidden' indicates the answer is hidden within the clue.
'EMPTY' is hidden within 'attempt you keep'.

(Other definitions for empty that I've seen before include "(Of words) lacking sincerity" , "Dead man" , "Containing nothing" , "Vacuous" , "Unoccupied" .)

I've seen this clue in The Times.

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