Road surfacing while parking has to stop (7)
I believe the answer is:
asphalt
'road surfacing' is the definition.
(I've seen this before)
'while parking has to stop' is the wordplay.
'while' becomes 'as' (eg 'while I walked' means 'as I walked').
'parking' becomes 'p' (abbreviation on road signs).
'has' says to put letters next to each other.
'to stop' becomes 'halt' (halt can mean to stop).
'as'+'p'+'halt'='ASPHALT'
(Other definitions for asphalt that I've seen before include "surface of path" , "Mixture of tar and gravel used in road surfaces" , "Substance used in roofing and paving" , "What you see on roads" , "Dark natural pitch" .)