Lettuce covered by sticky stuff: it's sweet (7)
I believe the answer is:
glucose
'it's sweet' is the definition.
(I know that glucose is sweet)
'lettuce covered by sticky stuff' is the wordplay.
'lettuce' becomes 'cos' (cos is a kind of lettuce).
'covered by' means one lot of letters goes inside another.
'sticky stuff' becomes 'glue' (stuff for sticking).
'cos' placed within 'glue' is 'GLUCOSE'.
(Other definitions for glucose that I've seen before include "An important source of body energy" , "Blood sugar" , "Sweet substance" , "something tasting sweet" , "Form of sugar found in fruit etc" .)