Highly recommends curses on heartless buddy (6,2)

I believe the answer is:

swears by

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'highly recommends' is the definition.
I can't judge whether this definition defines the answer.

'curses on heartless buddy' is the wordplay.
'curses' becomes 'swears' ('swear' can be a synonym of 'curse').
'on' is a charade indicator (letters next to each other).
'heartless' means to remove the middle letters.
'buddy' with its middle taken out is 'by'.
'swears'+'by'='SWEARS BY'

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

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