Mooring rope abaft grim way to lose weight (9)

I believe the answer is:

sternfast

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'mooring rope abaft' is the definition.
I know nothing about this answer so I cannot tell whether it can be defined by this definition.

'grim way to lose weight' is the wordplay.
'grim' becomes 'stern' (synonyms).
'way to lose weight' becomes 'fast' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'stern'+'fast'='STERNFAST'

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

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