Incentive in some useless purchase (4)

I believe the answer is:

spur

'incentive' is the definition.
(an incentive spur)

'in some useless purchase' is the wordplay.
'in' says the answer is hidden in the clue.
'SPUR' can be found hidden inside 'some useless purchase'.

(Other definitions for spur that I've seen before include "This provokes sudden movement!" , "Incite to action" , "Incentive; branch line" , "Hurry up" , "Stimulate" .)

I've seen this clue in The Times.

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