Almost decapitated sooner than expected (5)
I believe the answer is:
early
'sooner than expected' is the definition.
('early' can be a synonym of 'sooner than expected')
'almost decapitated' is the wordplay.
'almost' becomes 'nearly' ('nearly' can be a synonym of 'almost').
'decapitated' means to remove the first letter.
'nearly' with its first letter taken away is 'EARLY'.
(Other definitions for early that I've seen before include "Before appointed time" , "Before the ETA" , "Immature" , "Post this for Christmas?" , "Ahead of schedule" .)