Call staff about a bypass (4,4)
I believe the answer is:
ring road
'bypass' is the definition.
(I know that bypass can be written as ring road)
'call staff about a' is the wordplay.
'call' becomes 'ring' (telephone call).
'staff' becomes 'rod' (both can mean a stick or cane).
'about' is an insertion indicator.
'rod' enclosing 'a' is 'road'.
'ring'+'road'='RING-ROAD'
(Other definitions for ring road that I've seen before include "Route encircling a town" , "Way round a town" , "highway around town" , "roundabout?" , "Bypass encircling a town" .)