Flowering plant her many chums cultivated? About time (13)
I believe the answer is:
chrysanthemum
'flowering plant' is the definition.
(chrysanthemum is a kind of flowering plant)
'her many chums cultivated? about time' is the wordplay.
'cultivated?' indicates anagramming the letters.
'about' means one lot of letters goes inside another.
'time' becomes 't' (abbreviation).
'her'+'many'+'chums'='hermanychums'
'hermanychums' anagrammed gives 'chrysanhemum'.
'chrysanhemum' placed around 't' is 'CHRYSANTHEMUM'.
(Other definitions for chrysanthemum that I've seen before include "Common garden plant" , "Bright garden flower" , "Large, golden, daisy plant" , "Popular garden flower" , "Flower, literally golden one" .)