To some extent, ideas perplex Serpent (3)
I believe the answer is:
asp
'serpent' is the definition.
(asp is a kind of serpent)
'to some extent ideas perplex' is the wordplay.
'to some extent' indicates the answer is hidden within the clue.
'ASP' is hidden within 'ideas perplex'.
(Other definitions for asp that I've seen before include "Snake, killed Cleopatra" , "Serpent" , "Cleopatra's poisoner" , "something at the zoo" , "Slithery thing" .)