Change flag when heading off (5)
I believe the answer is:
alter
'change' is the definition.
('alter' can be a synonym of 'change')
'flag when heading off' is the wordplay.
'flag' becomes 'falter' (I've seen this before).
'when heading off' means to remove the first letter.
'falter' with its first letter removed is 'ALTER'.
(Other definitions for alter that I've seen before include "reform" , "Make changes to" , "and 10 Across: Second self" , "Become different" , "Revise" .)