Where housing’s lacking, inexperienced run fast (5,4)
I believe the answer is:
green belt
'where housing's lacking' is the definition.
(area which housing cannot be built on)
'inexperienced run fast' is the wordplay.
'inexperienced' becomes 'green' (similar in meaning).
'run fast' becomes 'belt' (to belt is to run very fast).
'green'+'belt'='GREEN BELT'
(Other definitions for green belt that I've seen before include "can't be built on?" , "Supposedly a no-go area for new building" , "Zone where development is banned (in theory)" , "protected area" , "area of open land near town" .)