Standard rule involving lairs (3,6)
I believe the answer is:
red ensign
'standard' is the definition.
(I've seen this before)
'rule involving lairs' is the wordplay.
'rule' becomes 'reign' (synonyms).
'involving' means one lot of letters goes inside another.
'lairs' becomes 'dens' ('den' can be a synonym of 'lair').
'reign' placed around 'dens' is 'RED ENSIGN'.
(Other definitions for red ensign that I've seen before include "standard for seamen" , "Flag for ships" , "Sailor's flag" , "It will fly at sea" , "Shipping flag" .)