Hard stone split by instant cut (6)
I believe the answer is:
strict
'hard' is the definition.
(I've seen this before)
'stone split by instant cut' is the wordplay.
'stone' becomes 'st'.
'split by' is an insertion indicator.
'instant' becomes 'trice' ('trice' can be a synonym of 'instant').
'cut' means to remove the last letter.
'trice' with its last letter taken away is 'tric'.
'st' going around 'tric' is 'STRICT'.
(Other definitions for strict that I've seen before include "Authoritarian" , "Observing rules very closely" , "Hard" , "Rigid or stern" , "Rigorous, uncompromising" .)