Constant battle over small wood (7)
I believe the answer is:
cypress
'wood' is the definition.
(cypress is a kind of wood)
'constant battle over small' is the wordplay.
'constant' becomes 'c' (mathematical abbreviation).
'battle' becomes 'ypres' (Ypres is an example**).
'over' says to put letters next to each other.
'small' becomes 's' (abbreviation - e.g. clothes size).
'c'+'ypres'+'s'='CYPRESS'
(Other definitions for cypress that I've seen before include "one rooted in the Med?" , "Evergreen conifer" , "Wood" , "One living in churchyard" , "Dark-leaved conifer" .)