Gabriel sees convert in headless man (9)
I believe the answer is:
archangel
'gabriel' is the definition.
(I've seen this before)
'convert in headless man' is the wordplay.
'convert' becomes 'change' (synonyms).
'in' is an insertion indicator.
'headless man' becomes 'arl' (I am not sure about this - if you are sure you should believe this answer much more).
'change' going into 'arl' is 'ARCHANGEL'.
'sees' acts as a link.
Can you help me to learn more?
(Other definitions for archangel that I've seen before include "Raphael, perhaps" , "Russian port on the White Sea" , "Among others, Michael" , "Nag Rachel (anag.)" , "Divine messenger - Russian port" .)