Regularly reuse, in gym, a training shoe (10)
I believe the answer is:
espadrille
'shoe' is the definition.
(I know that espadrille is a type of shoe)
'regularly reuse in gym a training' is the wordplay.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'in' means one lot of letters goes inside another.
'gym' becomes 'pe' (short for Physical Education).
'training' becomes 'drill' (drill is a kind of training).
The alternating letters of 'reuse' are 'es'.
'a'+'drill'='adrill'
'pe' enclosing 'adrill' is 'padrille'.
'es'+'padrille'='ESPADRILLE'
(Other definitions for espadrille that I've seen before include "Shoe with a canvas upper" , "Pale idlers (anag.)" , "Plaited fibres in its sole" , "Footwear" , "[SHOE], made casually" .)