Changed, or stumbled, heading off (7)
I believe the answer is:
altered
'changed' is the definition.
('alter' can be a synonym of 'change')
'stumbled heading off' is the wordplay.
'stumbled' becomes 'faltered' ('falter' can be a synonym of 'stumble').
'heading off' means to remove the first letter.
'faltered' with its first letter taken off is 'ALTERED'.
'or' acts as a link.
(Other definitions for altered that I've seen before include "Reformed" , "Converted" , "Change" , "Different" , "Changed, adapted" .)