. . .fighter: "just appalling!" (9)
I believe the answer is:
frightful
'appalling' is the definition.
(I've seen this before)
'fighter just' is the wordplay.
'fighter' becomes 'F' (abbreviation in war plane e.g. F-16 etc.).
'just' becomes 'rightful' (similar in meaning).
'f'+'rightful'='FRIGHTFUL'
(Other definitions for frightful that I've seen before include "from hell" , "ugly" , "savage" , "very bad" , "nasty" .)