Most horrible whopper punching guts naughtily (7)
I believe the answer is:
ugliest
'most horrible' is the definition.
('ugly' can be a synonym of 'horrible')
'whopper punching guts naughtily' is the wordplay.
'whopper' becomes 'lie' (I've seen this before).
'punching' is an insertion indicator (I've seen this in other clues).
'naughtily' is an anagram indicator.
'guts' is an anagram of 'ugst'.
'lie' placed inside 'ugst' is 'UGLIEST'.
(Other definitions for ugliest that I've seen before include "Most repulsive" , "most unappealing" , "Most unattractive in appearance" , "Least attractive" , "Least beautiful" .)