Calyx segment
I believe the answer is:
sepal
'calyx segment' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for sepal that I've seen before include "bit of a bloomer" , "Green part that forms a flower's calyx" , "Part of the calyx of a flower" , "Part of a flower - peals (anag)" , "Part of a flower's outer envelope" .)