Sum left Wendy regularly confused (6)
I believe the answer is:
addled
'confused' is the definition.
(addling is a kind of confusing)
'sum left wendy regularly' is the wordplay.
'sum' becomes 'add' ('add' can be a synonym of 'sum'**).
'left' becomes 'l' (common abbreviation).
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
The alternating letters of 'wendy' are 'ed'.
'add'+'l'+'ed'='ADDLED'
(Other definitions for addled that I've seen before include "Confused, mixed up like bad egg" , "Rotten; muddled" , "Dad led (anag.)" , "Lost" , "Confused and vague" .)