Like Jack Dawkins, Rex admitted to terrible fault (6)
I believe the answer is:
artful
'like jack dawkins' is the definition.
(I've seen this in another clue)
'rex admitted to terrible fault' is the wordplay.
'rex' becomes 'R' (abbreviation e.g. GR=George Rex, King George).
'admitted to' indicates putting letters inside.
'terrible' indicates anagramming the letters.
'fault' with letters rearranged gives 'atful'.
'r' going within 'atful' is 'ARTFUL'.
(Other definitions for artful that I've seen before include "Sly" , "Clever, tricky" , "Cunning, wily" , "Clever like Dickens's Dodger" , "Oliver Twist's pal was an . . . . . . dodger" .)