Bloomer's flatter crust's initially boring (9)
I believe the answer is:
buttercup
'bloomer's' is the definition.
(plant which blooms)
'flatter crust's initially boring' is the wordplay.
'flatter' becomes 'butterup' (buttering up is a kind of flattering).
'initially' indicates taking the first letters.
'boring' means one lot of letters goes inside another (to bore something can mean to drill into it).
The initial letter of 'crusts' is 'c'.
'butterup' going around 'c' is 'BUTTERCUP'.
(Other definitions for buttercup that I've seen before include "Common meadow flower" , "Rich yellow wild flower" , "growth in the field" , "Yellow-flowered grassland plant" , "Common grassland plant, poisonous to livestock" .)