Clue: Trees defining boundary of the Roman Empire (5)
Answer: LIMES
Explanation: Definition: Trees
Double definition: the Latin word Limes (related to "limit") can be used to describe to boundaries of the Roman empire, though it wasn't used in this sense by the Romans themselves. Perhaps a bit unfair to those who (like me) didn't know it, as PINES would fit just as well (and reminded me of Respighi's symphonic poem *Pines of Rome*)