To some extent problem is horrendous for Mark (7)
I believe the answer is:
blemish
'mark' is the definition.
(thesaurus)
'to some extent problem is horrendous' is the wordplay.
'to some extent' says the answer is hidden in the clue.
'BLEMISH' can be found hidden inside 'problem is horrendous'.
'for' is the link.
(Other definitions for blemish that I've seen before include "Noticeable flaw" , "deformity" , "Flaw; moral defect" , "Stain, flaw" , "Stain - defect" .)