Horrible to leave note without regrets? (8)
I believe the answer is:
gruesome
'horrible' is the definition.
(I've seen this before)
'leave note without regrets?' is the wordplay.
I cannot really see how this works, but
'note' could be 'so' (musical note) and 'so' is found in the answer.
This explanation may well be incorrect...
'to' is the link.
(Other definitions for gruesome that I've seen before include "Extremely unpleasant" , "Repugnant" , "Revolting, filling one with horror and disgust" , "'Ghastly, grisly (8)'" , "Go, resume being revolting or gruesome" .)