Cancelled rule, getting in difficulties about that (6)
I believe the answer is:
struck
'cancelled' is the definition.
(striking is a kind of cancelling)
'rule getting in difficulties about that' is the wordplay.
'rule' becomes 'r' (legal abbreviation).
'getting in difficulties' becomes 'stuck'.
'about that' is an insertion indicator.
'r' going within 'stuck' is 'STRUCK'.
(Other definitions for struck that I've seen before include "Given blows - affected instantaneously" , "hit hard" , "Hit or impressed" , "Took industrial action" , "stopped working" .)