Supports allowance for waste by church as part of the course (4,7)

I believe the answer is:

back stretch

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'the course' is the definition.
I know nothing about this answer so I cannot judge whether this works.

'supports allowance for waste by church as' is the wordplay.
'supports' becomes 'backs' (backing is a kind of supporting).
'allowance for waste' becomes 'tret' (I've seen this in another clue).
'by' is a charade indicator (letters next to each other).
'church as' becomes 'ch' (abbreviation on maps etc. I am not sure about the 'as' bit.).
'backs'+'tret'+'ch'='BACK STRETCH'

'part of' acts as a link.

Can you help me to learn more?

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

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