The sepals of a flower collectively (5)
I believe the answer is:
calyx
'the sepals of a flower collectively' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for calyx that I've seen before include "Sepals collectively" , "Protective layer around flower bud" , "Group of sepals" , "Part of a flower" , "A plant's natural cover" .)