Cut top of oak, cracking little wood (6)
I believe the answer is:
groove
'cut' is the definition.
(I know that groove is a more specific form of the action incise)
'top of oak cracking little wood' is the wordplay.
'top of' says to take the initial letters.
'cracking' indicates putting letters inside (inserted letters go into a gap or crack).
'little wood' becomes 'grove' (a grove is a small wood).
The initial letter of 'oak' is 'o'.
'o' inserted within 'grove' is 'GROOVE'.
(Other definitions for groove that I've seen before include "Long narrow cut in hard material" , "'Channel, furrow (6)'" , "Gramophone record track" , "Way of living that has become a habit" , "Established routine" .)