Area in which there may be no developments? (5,4)
I believe the answer is:
green belt
'area in which there may be no developments?' is the definition.
The definition suggests a singular noun which matches the answer.
'area in which' is the wordplay.
'area' becomes 'green' (green is a kind of area).
'in which' becomes 'belt' (I can't explain this - if you can you should believe this answer much more).
'green'+'belt'='GREEN BELT'
Can you help me to learn more?
(Other definitions for green belt that I've seen before include "Protected land around city" , "Something about London" , "status of karate?" , "land outside city" , "Circle determined to preserve" .)