Part of a flower calyx (5)
I believe the answer is:
sepal
'part of a flower calyx' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for sepal that I've seen before include "Lapse about part of a flower" , "Green part that forms a flower's calyx" , "Part of a flower - peals (anag)" , "Part of a flower casing" , "Part of a flowering plant" .)