Mum queueing where traffic's heaviest? (4,4)
I believe the answer is:
main line
'where traffic's heaviest?' is the definition.
(a main line is a major transport road or rail)
'mum queueing' is the wordplay.
'mum' becomes 'ma' (synonyms).
'queueing' becomes 'in line' (to queue is to wait in line).
'ma'+'inline'='MAIN LINE'
(Other definitions for main line that I've seen before include "Principal rouote of a transportation system" , "Way to travel" , "track on which many train" , "Inject drug - principal rail connection (8;4,4)" , "Take 9 intravenously" .)