Crossword Genius

To split hairs can upset something ground-breaking (3-4)

Ross

I believe the answer is:

nit-pick

'to split hairs' is the definition.
(both can mean to pedantically criticise)

'can upset something ground-breaking' is the wordplay.
'can' becomes 'tin' (synonyms).
'upset' shows that the letters should be reversed in order.
'something ground-breaking' becomes 'pick' (a miner might use a pick to break into the ground).
'tin' reversed gives 'nit'.
'nit'+'pick'='NIT-PICK'

I've seen this clue in The Times.
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