Hard Viking decapitated rider (8)
I believe the answer is:
horseman
'rider' is the definition.
(someone riding a horse)
'hard viking decapitated' is the wordplay.
'hard' becomes 'h' (abbreviation used in pencil classifications).
'viking' becomes 'norseman' (I've seen this in another clue).
'decapitated' means to remove the first letter.
'norseman' with its first letter taken away is 'orseman'.
'h'+'orseman'='HORSEMAN'
(Other definitions for horseman that I've seen before include "mounted rider" , "Equestrian" , "(Skilled) rider" , "Male rider" , "Jockey" .)