Plant thyme with March sun about (13)
I believe the answer is:
chrysanthemum
'plant' is the definition.
(chrysanthemum is a kind of plant)
'thyme with march sun about' is the wordplay.
'with' says to put letters next to each other.
'about' is an anagram indicator.
'march'+'sun'='marchsun'
'thyme' put after 'marchsun' is 'marchsunthyme'.
'marchsunthyme' is an anagram of 'CHRYSANTHEMUM'.
(Other definitions for chrysanthemum that I've seen before include "Bright garden flower" , "Common garden flower" , "Large, golden, daisy plant" , "Perennial" , "Flower, literally golden one" .)