Flower part is close protecting head of plant (5)
I believe the answer is:
sepal
'flower part' is the definition.
(I've seen this before)
'close protecting head of plant' is the wordplay.
'close' becomes 'seal' (sealing is a kind of closing**).
'protecting' is an insertion indicator.
'head of' indicates taking the first letters.
The initial letter of 'plant' is 'p'.
'seal' enclosing 'p' is 'SEPAL'.
'is' is the link.
(Other definitions for sepal that I've seen before include "Part of a flower casing" , "Violet" , "flowery bit" , "Part of a flower's outer envelope" , "Calyx segment" .)