Overhasty, forward missing header (4)
I believe the answer is:
rash
'overhasty' is the definition.
(I've seen this before)
'forward missing header' is the wordplay.
'forward' becomes 'brash' (both can mean confident or arrogant).
'missing header' means to remove the first letter.
'brash' with its first letter removed is 'RASH'.
(Other definitions for rash that I've seen before include "Over-hasty, reckless" , "Careless" , "Hives" , "sign of irritation?" , "Outbreak; hasty" .)