Girl beheaded Danish king Filbert (8)
I believe the answer is:
hazelnut
'filbert' is the definition.
(synonyms)
'girl beheaded danish king' is the wordplay.
'girl' becomes 'Hazel' (girl's name).
'beheaded' means to remove the first letter (the head of the word is removed).
'danish king' becomes 'cnut' (King Cnut, 10th- and 11th-century king).
'cnut' with its initial letter taken off is 'nut'.
'hazel'+'nut'='HAZELNUT'
(Other definitions for hazelnut that I've seen before include "Cobnut or filbert" , "Filbert or cob" .)