Sticky stuff covering lettuce is sugar (7)
I believe the answer is:
glucose
'is sugar' is the definition.
(I've seen this before)
'sticky stuff covering lettuce' is the wordplay.
'sticky stuff' becomes 'glue' (I've seen this before).
'covering' is an insertion indicator.
'lettuce' becomes 'cos' (cos lettuce is a kind of lettuce).
'glue' enclosing 'cos' is 'GLUCOSE'.
(Other definitions for glucose that I've seen before include "something tasting sweet" , "Form of sugar found in fruit etc" , "Blood sugar" , "An important source of body energy" , "syrupy stuff" .)