Crafty Rex admitted to fault – that's unusual (6)
I believe the answer is:
artful
'crafty' is the definition.
('artful' can be similar in meaning to 'crafty')
'rex admitted to fault that's unusual' is the wordplay.
'rex' becomes 'r' (common abbreviation e.g. GR=George Rex, King Georg).
'admitted to' is an insertion indicator.
'that's unusual' indicates an anagram.
'fault' is an anagram of 'atful'.
'r' going into 'atful' is 'ARTFUL'.
(Other definitions for artful that I've seen before include "Unnatural - cunning" , "Dickens boy, an . . . . . . Dodger" , "Crafty like Dicken's Dodger" , "Clever, tricky" , "Sly" .)