Tree in WWI battle swathed in gas (7)
I believe the answer is:
cypress
'tree' is the definition.
(cypress tree is a kind of tree)
'wwi battle swathed in gas' is the wordplay.
'wwi battle' becomes 'ypres' (Battle of Ypres, 1918 battle).
'swathed in' means one lot of letters goes inside another (swathe can mean to wrap).
'gas' becomes 'cs' (short for 2-chlorobenzalmalononitrile).
'ypres' going within 'cs' is 'CYPRESS'.
'in' acts as a link.
(Other definitions for cypress that I've seen before include "symbol of death" , "Evergreen conifer" , "Conifer, symbol of mourning" , "one rooted in the Med?" , "Dark-leaved conifer" .)