Detesting hard, giant puzzle (6)
I believe the answer is:
hating
'detesting' is the definition.
('hate' can be a synonym of 'detest')
'hard giant puzzle' is the wordplay.
'hard' becomes 'h' (abbreviation used in pencil classifications).
'puzzle' indicates an anagram.
'giant' with letters rearranged gives 'ating'.
'h'+'ating'='HATING'
(Other definitions for hating that I've seen before include "Abhorring" , "Having an aversion to" , "Loathing" , "Disliking intensely" , "Detesting" .)