Environmental concern, a constant international depletion of resources (4,4)
I believe the answer is:
acid rain
'environmental concern' is the definition.
(I've seen this in another clue)
'a constant international depletion of resources' is the wordplay.
'constant' becomes 'C' (mathematical abbreviation).
'international' becomes 'i' (abbreviation eg in IMF).
'depletion of resources' becomes 'drain' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'a'+'c'+'i'+'drain'='ACID-RAIN'
Can you help me to learn more?
(Other definitions for acid rain that I've seen before include "Destructive precipitation" , "environmental disaster" , "pollution falling" , "Harmful precipitation" , "Type of pollution" .)