Labour's difficult operation (4,5)

I believe the answer is:

hard graft

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'operation' is the definition.
I don't know anything about this answer so I can't judge whether it can be defined by this definition.

'labour's difficult' is the wordplay.
I cannot really see how this works, but
'difficult' could be 'hard' (synonyms) and 'hard' is present in the answer.
'labour' could be 'graft' (I've seen this before) and 'graft' is located in the answer.
No letters remain.
This explanation may well be incorrect...

Can you help me to learn more?

(Another definition for hard graft that I've seen is " Demanding work".)

I've seen this clue in the Evening Standard.

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