Regrets delving into work, this writer's terrible (8)
I believe the answer is:
gruesome
'terrible' is the definition.
(thesaurus)
'regrets delving into work this writer's' is the wordplay.
'regrets' becomes 'rues' ('rue' can be a synonym of 'regret').
'delving into' means one lot of letters goes inside another.
'work' becomes 'go' ('go' can be a synonym of 'work').
'this writer' becomes 'me' (the person who is writing these clues).
'rues' going inside 'go' is 'grueso'.
'grueso'+'me'='GRUESOME'
(Other definitions for gruesome that I've seen before include "Grisly, horrible" , "Go, resume being revolting or gruesome" , "Horrifying and revolting" , "Repulsive, horrid" , "Extremely unpleasant" .)