Lacking vigour, trainer takes part (5)

I believe the answer is:

inert

'lacking vigour' is the definition.
(I've seen this before)

'trainer takes part' is the wordplay.
'part' indicates the answer is hidden within the clue.
'INERT' is hidden within 'trainer takes'.

(Other definitions for inert that I've seen before include "Like e.g. xenon" , "Leaden" , "Passive, immobile" , "Not active or reactive" , "Sluggish - unresponsive" .)

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