Deadly charm, fulsome to some extent (7)
I believe the answer is:
harmful
'deadly' is the definition.
('harmful' can be similar in meaning to 'deadly')
'charm fulsome to some extent' is the wordplay.
'to some extent' says the answer is hidden in the clue.
'HARMFUL' is hidden in the letters of 'charm fulsome'.
(Other definitions for harmful that I've seen before include "Noxious" , "Causing injury or damage, say poison or malice" , "Dangerous, damaging" , "Deleterious" , "Injurious" .)