Clue: British on getting into vehicle slump, it may save the planet (6,4)
Answer: CARBON SINK
Explanation: Definition: it may save the planet

**B** (British) + **ON ***getting into* **CAR **("vehicle") **SINK **("slump")
 
*"Carbon sink" is a generic term for a reservoir that absorbs more carbon that it emits, such as the ocean or a rainforest.