One practising with principal coach, not second (10)

I believe the answer is:

maintainer

'one practising' is the definition.
(someone who practises or maintains something)

'principal coach not second' is the wordplay.
I cannot really see how this works, but
'principal' could be 'main' (both can mean major) and 'main' is found within the answer.
This explanation may well be incorrect...

'with' is the link.

(Another definition for maintainer that I've seen is " Supporter".)

I've seen this clue in The Times.

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