Sound disapproving of motorway expansion (4)
I believe the answer is:
boom
'expansion' is the definition.
(period of economic growth)
'sound disapproving of motorway' is the wordplay.
'sound disapproving of' becomes 'boo' (to boo is to audibly disapprove of something).
'motorway' becomes 'M' (as in motorway names - M1, M4 etc).
'boo'+'m'='BOOM'
(Other definitions for boom that I've seen before include "Pick up big" , "Crash" , "sound of artillery fire" , "pole attached to sail" , "Bonanza; roar" .)