Almost beheaded — before dawn? (5)
I believe the answer is:
early
'before dawn?' is the definition.
(I've seen this in another clue)
'almost beheaded' is the wordplay.
'almost' becomes 'nearly' (synonyms).
'beheaded' means to remove the first letter (the head of the word is removed).
'nearly' with its first letter removed is 'EARLY'.
(Other definitions for early that I've seen before include "before anyone else?" , "Forward" , "sooner than expected" , "Before the ETA" , "Premature" .)