Something sweet to stick around lettuce (7)
I believe the answer is:
glucose
'something sweet' is the definition.
(type of sugar)
'stick around lettuce' is the wordplay.
'stick' becomes 'glue' (I've seen this before).
'around' is an insertion indicator.
'lettuce' becomes 'cos' (cos is a kind of lettuce).
'glue' going around 'cos' is 'GLUCOSE'.
'to' acts as a link.
(Other definitions for glucose that I've seen before include "An important source of body energy" , "Naturally occurring sugar" , "Simple sugar" , "Blood sugar" , "Sweet substance" .)