Sugar is adhesive around lettuce (7)
I believe the answer is:
glucose
'sugar' is the definition.
(type of sugar)
'adhesive around lettuce' is the wordplay.
'adhesive' becomes 'glue' (glue is a kind of adhesive).
'around' is an insertion indicator.
'lettuce' becomes 'cos' (cos is a kind of lettuce).
'glue' enclosing 'cos' is 'GLUCOSE'.
'is' is the link.
(Other definitions for glucose that I've seen before include "Form of sugar found in fruit etc" , "Sweet substance" , "something tasting sweet" , "Type of sugar" , "An important source of body energy" .)