Almost heading off too soon (5)
I believe the answer is:
early
'too soon' is the definition.
(earlier than required)
'almost heading off' is the wordplay.
'almost' becomes 'nearly' (synonyms).
'heading off' means to remove the first letter.
'nearly' with its initial letter taken off is 'EARLY'.
(Other definitions for early that I've seen before include "Forward" , "Before appointed time" , "Before due time" , "Immature" , "Before the expected time" .)