In the morning, bus dropping back behind schedule — walk! (8)
I believe the answer is:
ambulate
'walk' is the definition.
(ambulating is a kind of walking)
'in the morning bus dropping back behind schedule' is the wordplay.
'in the morning' becomes 'am'.
'dropping back' means to remove the last letter.
'behind schedule' becomes 'late' (I've seen this before).
'bus' with its last letter removed is 'bu'.
'am'+'bu'+'late'='AMBULATE'
(Other definitions for ambulate that I've seen before include "Go for a stroll" , "Walk (technical)" .)