Proficient fighter for restricted building area (5,4)
I believe the answer is:
green belt
'proficient fighter' is the definition.
The definition suggests a singular noun which matches the answer.
'restricted building area' is the wordplay.
I cannot quite understand how this works, but
'area' could be 'green' (green is a kind of area) and 'green' is present in the answer.
The remaining letters 'belt' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
'for' acts as a link.
Can you help me to learn more?
(Other definitions for green belt that I've seen before include "Protected land around city" , "of restricted development?" , "status of karate?" , "Supposedly a no-go area for new building" , "land outside city" .)