Sort of gas enshrouding Belgian battlefield tree (7)
I believe the answer is:
cypress
'tree' is the definition.
(cypress is a kind of tree)
'sort of gas enshrouding belgian battlefield' is the wordplay.
'sort of gas' becomes 'CS' (short for 2-chlorobenzalmalononitrile).
'enshrouding' means one lot of letters goes inside another (enshroud can mean to cover up or conceal).
'belgian battlefield' becomes 'Ypres' (site of the Battle of Ypres, 1918).
'cs' enclosing 'ypres' is 'CYPRESS'.
(Other definitions for cypress that I've seen before include "Evergreen, mourning symbol" , "Widely cultivated conifer" , "Tree with funereal associations" , "Coniferous tree" , "Conifer, symbol of mourning" .)