Sluggish trainer tires somewhat (5)

I believe the answer is:

inert

'sluggish' is the definition.
('inert' can be a synonym of 'sluggish')

'trainer tires somewhat' is the wordplay.
'somewhat' says the answer is hidden in the clue.
'INERT' is hidden in the letters of 'trainer tires'.

(Other definitions for inert that I've seen before include "Lifeless; apathetic" , "Nothing doing" , "Devoid of motion or change" , "Unable to move or resist motion" , "Like some gases" .)

I've seen this clue in The Telegraph.

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