Spring training in extremely effective footwear (10)

I believe the answer is:

espadrille

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'footwear' is the definition.
(espadrille is a kind of footwear)

'spring training in extremely effective' is the wordplay.
'spring' becomes 'spa' (I've seen this before).
'training' becomes 'drill' (drill is a kind of training).
'in' indicates putting letters inside.
'extremely effective' becomes 'ee' (this could be a standard abbreviation I've not previously seen).
'spa'+'drill'='spadrill'
'spadrill' placed into 'ee' is 'ESPADRILLE'.

Can you help me to learn more?

(Other definitions for espadrille that I've seen before include "Pale idlers (anag.)" , "Plaited fibres in its sole" , "[SHOE], made casually" , "Footwear" , "sandal!" .)

I've seen this clue in The Independent.

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