Behind schedule boarding coach after morning walks (9)

'walks' is the definition.
(ambulating is a kind of walking)

'behind schedule boarding coach after morning' is the wordplay.
'behind schedule' becomes 'late' (I've seen this before).
'boarding' indicates putting letters inside.
'coach' becomes 'bus' (synonyms).
'after' means one lot of letters go next to another.
'morning' becomes 'am' (as in bu:00 AM).
'late' put inside 'bus' is 'bulates'.
'bulates' put after 'am' is 'AMBULATES'.