Fern Britton's head stand when half-cut (7)
I believe the answer is:
bracken
'fern' is the definition.
(type of fern)
'britton's head stand when half-cut' is the wordplay.
'head' says to take the initial letters.
'stand' becomes 'rack' (synonyms).
'half cut' suggests halving the letters.
The initial letter of 'britton' is 'b'.
'when' cut in half is 'en'.
'b'+'rack'+'en'='BRACKEN'
(Other definitions for bracken that I've seen before include "British moorland fern" , "Mass of ferns as on mountains" , "Coarse moorland fern" , "Coarse ferns" , "Fern on the hillside" .)