Cancel first of rules in facility (5)
I believe the answer is:
erase
'cancel' is the definition.
(I know that erase is a type of cancel)
'first of rules in facility' is the wordplay.
'first of rules' becomes 'r' (est letter of 'rules').
'in' indicates putting letters inside.
'facility' becomes 'ease' (I've seen this in another clue).
'r' put into 'ease' is 'ERASE'.
(Other definitions for erase that I've seen before include "Kill" , "Take out" , "Remove, wipe out" , "Rub out, cancel" , "Clear" .)