Start to make everybody walk (4)
'walk' is the definition.
(mall is a kind of walk)
'start to make everybody' is the wordplay.
'start to' indicates taking the first letters.
'everybody' becomes 'all' (I've seen this before).
The first letter of 'make' is 'm'.
(Other definitions for mall that I've seen before include "Shopping area closed to vehicles", "Pedestrian shopping area", "Shopping facility", "Traffic-free street", "Covered shopping centre".)