Peas cooked with large part of flower (5)
I believe the answer is:
sepal
'part of flower' is the definition.
(I know that calyx is a is part of of sepal)
'peas cooked with large' is the wordplay.
'cooked' indicates anagramming the letters (letters cooked into a new form).
'with' is a charade indicator (letters next to each other).
'large' becomes 'l' (eg in clothes sizes).
'peas' anagrammed gives 'sepa'.
'sepa'+'l'='SEPAL'
(Other definitions for sepal that I've seen before include "Green part that forms a flower's calyx" , "Enclosing part of flower" , "bit of a bloomer" , "Part of a flower's outer envelope" , "Lapse about part of a flower" .)