Mist lying on top of locust tree (5)
I believe the answer is:
hazel
'tree' is the definition.
(hazel tree is a kind of tree)
'mist lying on top of locust' is the wordplay.
'mist' becomes 'haze' (I've seen this before).
'lying on' means one lot of letters go next to another.
'top of' says to take the initial letters.
The first letter of 'locust' is 'l'.
'haze'+'l'='HAZEL'
(Other definitions for hazel that I've seen before include "Girl" , "Tree - greenish-brown" , "Catkin-bearing tree" , "Shrub called the cob, which yields nuts" , "Tree of the birch family" .)