Bog covering head of Robert Plant (4)
I believe the answer is:
fern
'plant' is the definition.
(I know that fern is a type of nonflowering plant)
'bog covering head of robert' is the wordplay.
'bog' becomes 'fen' (I've seen this in another clue).
'covering' indicates putting letters inside.
'head of' suggests taking the first letters.
The first letter of 'robert' is 'r'.
'fen' enclosing 'r' is 'FERN'.
(Other definitions for fern that I've seen before include "Girl - bracken" , "Plant such as bracken" , "Non-flowering plant" , "Type of plant" , "Feathery-leaved plant" .)