What might be a warning device for the plant (8)
I believe the answer is:
Here is my best explanation:
'the plant' is the definition.
The answer is an example.
'what might be a warning device' is the wordplay.
'might be' is an anagram indicator.
'a warning device' becomes 'horn' (horn is a kind of warning device).
'what' is an anagram of 'hawt'.
'for' is the link.
(Other definitions for hawthorn that I've seen before include "May-flowering small tree", "Hedge-tree", "Small spring-blooming tree", "Spring-flowering small tree with small dark red fruit", "fruit supplier", "Bush", "Hedge tree", "Spiky tree with small red berries", "A small tree" and "Spring-flowering shrub".)