Clue: OECD chief explodes at national leaders as this rises? (5)
Answer: OCEAN
Explanation: Definition: as this rises
 **1st letters, respectively, of**(… leaders) "OECD chief explodes at national".
Defn: "this" in the defn.
(And, the OECD (Organisation for Economic Co-operation and Development) assresses, among other things, climate change, an effect of which is the rise in sea level as a result of global warming.)