Head of serpent mostly emulating Adam and Eve in Eden? Wily thing (5)
I believe the answer is:
snake
'wily thing' is the definition.
(snakes are proverbially wily or deceitful)
'head of serpent mostly emulating adam and eve in eden?' is the wordplay.
'head of' indicates taking the first letters.
'mostly' means to remove the last letter.
'emulating adam and eve in eden?' becomes 'naked' (Adam and Eve were naked in the Garden of Eden).
The first letter of 'serpent' is 's'.
'naked' with its last letter taken off is 'nake'.
's'+'nake'='SNAKE'
(Other definitions for snake that I've seen before include "Part of plant or hair" , "Move sinuously" , "Long slender reptile" , "Wind" , "Limbless reptile" .)