Tree in mist by lake (5)
I believe the answer is:
hazel
'tree' is the definition.
(hazel is a kind of tree)
'mist by lake' is the wordplay.
'mist' becomes 'haze' (haze can mean a mist).
'by' means one lot of letters go next to another.
'lake' becomes 'L' (geographical abbreviation).
'haze'+'l'='HAZEL'
'in' acts as a link.
(Other definitions for hazel that I've seen before include "Catkin shrub" , "Tree with catkins" , "Nut tree" , "Nut-bearing tree" , "Nut; girl's name" .)