Casually sleep with English scruff (4)
I believe the answer is:
nape
'scruff' is the definition.
('nape' can be a synonym of 'scruff')
'casually sleep with english' is the wordplay.
'casually sleep' becomes 'nap'.
'with' means one lot of letters go next to another.
'english' becomes 'e' (abbreviation).
'nap'+'e'='NAPE'
(Other definitions for nape that I've seen before include "sofa" , "Back part of the neck" , "Girl personifying France" , "It's a pane in the neck" , "Here's a hairline" .)