To split hairs can upset something ground-breaking (3-4)
I believe the answer is:
'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'.