Joining but, at start of training, suffering some pain (9)

I believe the answer is:

attaching

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'joining' is the definition.
(attaching is a kind of joining)

'at start of training suffering some pain' is the wordplay.
'start of' suggests taking the first letters.
'suffering some pain' becomes 'aching' (I can't explain this - if you can you should believe this answer much more).
The initial letter of 'training' is 't'.
'at'+'t'+'aching'='ATTACHING'

'but' acts as a link.

Can you help me to learn more?

(Another definition for attaching that I've seen is " Joining, fastening".)

I've seen this clue in the Sydney Morning Herald.

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