Monster with New Age variety flower (9)
I believe the answer is:
hydrangea
'flower' is the definition.
(I've seen this before)
'monster with new age variety' is the wordplay.
'monster' becomes 'hydra' (Hydra is an example).
'with' indicates putting letters inside.
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'variety' indicates an anagram.
'age' anagrammed gives 'gea'.
'n' inserted within 'gea' is 'ngea'.
'hydra'+'ngea'='HYDRANGEA'
(Other definitions for hydrangea that I've seen before include "Shrub - garden hay (anag)" , "Flowering plant - angry head (anag)" , "Ornamental flowering shrub - handy gear (anag)" , "Garden flower - garden hay (anag)" , "Shrub of the saxifrage family -- angry head (anag)" .)