Rule about changing ends for flag (3,6)
I believe the answer is:
red ensign
'flag' is the definition.
(thesaurus)
'rule about changing ends' is the wordplay.
'rule' becomes 'reign' (reign is a kind of rule**).
'about' indicates putting letters inside.
'changing' is an anagram indicator.
'ends' is an anagram of 'dens'.
'reign' enclosing 'dens' is 'RED ENSIGN'.
'for' acts as a link.
(Other definitions for red ensign that I've seen before include "Flag flown by the British Merchant Navy" , "Shipping flag" , "It will fly at sea" , "Flag for ships" , "standard for seamen" .)