Fern "seed" (5)
I believe the answer is:
spore
'fern seed' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for spore that I've seen before include "Reproductive body produced by fungi" , "Fungal cell" , "Fungus reproducer" , "Tiny reproductive cell of fungus or fern" , "Asexual reproductive body" .)