Change flag, heading off (5)
I believe the answer is:
alter
'change' is the definition.
('alter' can be a synonym of 'change')
'flag heading off' is the wordplay.
'flag' becomes 'falter' (I've seen this before).
'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 "Metamorphose" , "and 10 Across: Second self" , "Change, make different" , "Vary" , "Make changes to" .)