Flower spread, a hundred having risen (9)
I believe the answer is:
buttercup
'flower' is the definition.
(thesaurus)
'spread a hundred having risen' is the wordplay.
'spread' becomes 'butter' (I've seen this before**).
'a hundred' becomes 'c' (roman numeral).
'having' means one lot of letters go next to another.
'risen' becomes 'up' ('up' can be similar in meaning to 'risen').
'butter'+'c'+'up'='BUTTERCUP'
(Other definitions for buttercup that I've seen before include "Common grassland plant, poisonous to livestock" , "Meadow flower poisonous to cattle" , "Goldilocks, possibly" , "Yellow-flowered grassland plant" , "Common meadow flower" .)