Gentle exercise starts to add tone, stretching a lot (7)

I believe the answer is:

pilates

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'gentle exercise' is the definition.
'pilates' can be an answer for 'exercise' (pilates is a type of exercise). I'm unsure of the 'gentle' bit.

I don't understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for pilates that I've seen before include "attributed to Biblical procurator?" , "Exercise system" , "Set of exercises used in holistic medicine" , "Modern exercise" , "Holistic system of physical exercises" .)

I've seen this clue in The Independent.

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