Flower barrel, one hidden by legume
I believe the answer is:
petunia
'flower' is the definition.
(petunia is a kind of flower)
'barrel one hidden by legume' is the wordplay.
'barrel' becomes 'tun' (tun is a kind of barrel).
'one' becomes 'i' (Roman numeral).
'hidden by' is an insertion indicator.
'legume' becomes 'pea' (pea is a kind of legume).
'tun'+'i'='tuni'
'tuni' going into 'pea' is 'PETUNIA'.
(Other definitions for petunia that I've seen before include "Auntie foolishly made under pressure" , "Plant cultivated for its white, pink, blue or purple funnel-shaped flowers" , "Colourful" , "Garden plant with large funnel-shaped flower" , "Flowering plant" .)