Stick lettuce inside something sweet (7)
I believe the answer is:
glucose
'something sweet' is the definition.
(type of sugar)
'stick lettuce inside' is the wordplay.
'stick' becomes 'glue' (to stick with glue).
'lettuce' becomes 'cos' (cos is a kind of lettuce).
'inside' indicates putting letters inside.
'glue' going around 'cos' is 'GLUCOSE'.
(Other definitions for glucose that I've seen before include "something tasting sweet" , "An important source of body energy" , "Simple sugar" , "Sweet substance" , "Form of sugar found in fruit etc" .)