Backer mostly into monster plot feature (9)
I believe the answer is:
hydrangea
'plot feature' is the definition.
(might feature in a garden plot)
'backer mostly into monster' is the wordplay.
'backer' becomes 'angel' ('angel' can be a synonym of 'backer').
'mostly' means to remove the last letter.
'into' indicates putting letters inside.
'monster' becomes 'hydra' (Hydra is an example**).
'angel' with its final letter removed is 'ange'.
'ange' going into 'hydra' is 'HYDRANGEA'.
(Other definitions for hydrangea that I've seen before include "Ornamental flowering shrub - handy gear (anag)" , "Shrubby plant with showy flowers" , "Shrub of the saxifrage family -- angry head (anag)" , "This flower is strangely a hay danger" , "Common garden plant" .)