One will be sorry to do it (6)
I believe the answer is:
grieve
'to do it' is the definition.
Although both the answer and definition are verbs in their base form, I don't understand how one could define the other.
'one will be sorry' is the wordplay.
I cannot really see how this works, but
'one' could be 'i' (Roman numeral) and 'i' is located in the answer.
The remaining letters 'greve' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for grieve that I've seen before include "Suffer great sorrow" , "Cause great distress to" , "Feel deep sorrow as for a death" , "Mourn loss" , "Mourn, feel sorrow" .)