Force unit to cover wide urban development (3,4)
I believe the answer is:
new town
'urban development' is the definition.
(I've seen this before)
'force unit to cover wide' is the wordplay.
'force unit' becomes 'newton' (newton is a kind of force unit).
'to cover' means one lot of letters goes inside another.
'wide' becomes 'w' (abbreviation e.g. in cricket).
'newton' going around 'w' is 'NEW TOWN'.
(Other definitions for new town that I've seen before include "Stevenage, for example" , "Planned settlement to relieve congestion" , "Place for population overspill?" , "specially-built dwelling-place" , "garden city?" .)