One king is showing the flag (4)
I believe the answer is:
iris
'the flag' is the definition.
('iris' can be a synonym of 'flag')
'one king is' is the wordplay.
'one' becomes 'i' (Roman numeral).
'king' becomes 'r' (r is abbreviation for rex).
'i'+'r'+'is'='IRIS'
'showing' is the link.
(Other definitions for iris that I've seen before include "Rainbow - flag" , "--- Murdoch, author" , "Flower - girl's name" , "Flower and eye membrane" , "- - Murdoch, novelist" .)