Old region sounds complete environmental problem (5,4)
I believe the answer is:
ozone hole
'environmental problem' is the definition.
I don't know anything about this answer so I cannot judge whether it can be defined by this definition.
'old region sounds complete' is the wordplay.
'old region' becomes 'ozone' (I am not sure about this - if you are sure you should believe this answer much more).
'sounds' shows a homophone (sound like).
'complete' becomes 'whole' (similar in meaning).
'whole' is a homophone of 'hole'.
'ozone'+'hole'='OZONE HOLE'
Can you help me to learn more?
(Other definitions for ozone hole that I've seen before include "very high deficit" , "Thinning of the atmosphere at high latitudes" , "Depleted layer in the atmosphere near the poles" , "It increases hazardous burn rate" , "form of atmospheric disturbance" .)