Replaced a grim, long-established flower (8)
I believe the answer is:
marigold
'flower' is the definition.
(marigold is a kind of flower)
'replaced a grim long-established' is the wordplay.
'replaced' indicates anagramming the letters.
'long-established' becomes 'old' (I've seen this before).
'a'+'grim'='agrim'
'agrim' is an anagram of 'marig'.
'marig'+'old'='MARIGOLD'
(Other definitions for marigold that I've seen before include "Garden plant of the daisy family" , "Yellow or orange flower" , "Bright orange-yellow garden flower" , "Orange-flowered plant" , "The flower has a gold rim, oddly" .)