A mostly concrete neighbourhood (4)
I believe the answer is:
area
'neighbourhood' is the definition.
(I've seen this before)
'a mostly concrete' is the wordplay.
'mostly' means to remove the last letter.
'concrete' becomes 'real' (associated in meaning).
'real' with its last letter removed is 'rea'.
'a'+'rea'='AREA'
(Other definitions for area that I've seen before include "environs" , "Expanse or neighbourhood" , "Expanse of a surface" , "Succession, order" , "Extent of a 2-dimensional surface" .)