Lettuce covered in sticky stuff (sugar) (7)
I believe the answer is:
glucose
'sugar' is the definition.
(I've seen this before)
'lettuce covered in sticky stuff' is the wordplay.
'lettuce' becomes 'cos' (cos lettuce is a kind of lettuce).
'covered in' is an insertion indicator.
'sticky stuff' becomes 'glue' (I've seen this before).
'cos' put inside 'glue' is 'GLUCOSE'.
(Other definitions for glucose that I've seen before include "Type of sugar" , "An important source of body energy" , "syrupy stuff" , "Simple sugar" , "Form of sugar found in fruit etc" .)