Tying things together, building one’s body up, avoiding some treats — for fitness ultimately! (9)

I believe the answer is:

fastening

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

I cannot understand how the remainder of the clue works.

(Other definitions for fastening that I've seen before include "Device that closes or secures" , "Tying" , "Securing device" .)

I've seen this clue in The Hindu.

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