A northerner brought up fish that's frightful (9)
I believe the answer is:
appalling
'frightful' is the definition.
(thesaurus)
'a northerner brought up fish' is the wordplay.
'northerner' becomes 'lapp' (I have seen 'Far northerner' mean 'lapp' so perhaps 'northerner' could also mean 'lapp').
'brought up' shows that the letters should be reversed in order (in a down clue, letters go up).
'fish' becomes 'ling' (ling is a kind of fish).
'lapp' written backwards gives 'ppal'.
'a'+'ppal'+'ling'='APPALLING'
'that's' acts as a link.
(Other definitions for appalling that I've seen before include "page entirely covered in copying" , "Greatly horrifying" , "Awful" , "Shockingly bad" , "Dreadful" .)