Lack vehicle in south London, say (8)
I believe the answer is:
scarcity
'lack' is the definition.
(thesaurus)
'vehicle in south london say' is the wordplay.
'vehicle' becomes 'car' (car is a kind of vehicle).
'in' means one lot of letters goes inside another.
'south' becomes 's' ('S' can be a synonym of 'south').
'london say' becomes 'city' (london is a kind of city).
's'+'city'='scity'
'car' put within 'scity' is 'SCARCITY'.
(Other definitions for scarcity that I've seen before include "Shortness of supply" , "Want" , "Something uncommon" , "Shortage" , "Poverty" .)