Flower is good in territory’s first flag (6)
I believe the answer is:
tigris
'flower' is the definition.
(Tigris is an example)
'good in territory's first flag' is the wordplay.
'good' becomes 'g' (abbreviation).
'in' is an insertion indicator.
'territory's first' becomes 't' (1st letter of 'territory').
'flag' becomes 'iris' (type of iris flower).
't'+'iris'='tiris'
'g' put within 'tiris' is 'TIGRIS'.
'is' is the link.
(Other definitions for tigris that I've seen before include "Flower of Baghdad" , "It flows from Turkey to the Persian Gulf" , "Sit rig into the Asian river" , "Great river of 30 across" , "Great river flowing from Turkey through Iraq" .)