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 subsidiary indication.
'might be' is an anagram indicator.
'a warning device' becomes 'horn' (horn is a kind of warning device).
'what' with letters rearranged gives 'hawt'.
'for' is the link.
(Other definitions for hawthorn that I've seen before include "Spiky tree with small red berries", "Tree", "Spring-flowering shrub", "Spring-flowering small tree with small dark red fruit", "Hedge-tree", "May-flowering small tree", "Prickly shrub", "Common hedge bush", "Hedgerow bush" and "May tree".)