It's not reasonable for individual priest to take head off citizen (10)
I believe the answer is:
irrational
'it's not reasonable' is the definition.
(I've seen this in another clue)
'individual priest to take head off citizen' is the wordplay.
'individual priest' becomes 'irr' (I can't justify this - if you can you should give a lot more credence to this answer).
'to take head off' means to remove the first letter (I've seen 'taking head off' mean this).
'citizen' becomes 'national' (national can mean a citizen of a country).
'national' with its first letter taken away is 'ational'.
'irr'+'ational'='IRRATIONAL'
'for' is the link.
Can you help me to learn more?
(Other definitions for irrational that I've seen before include "not sensible" , "Crazy" , "Unreasonable" , "Not logical" , "nuts" .)