Think capital offence does require beheading (6)
I believe the answer is:
reason
'think' is the definition.
(reasoning is a kind of thinking)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for reason that I've seen before include "Power of thought" , "issue" , "Cause - logic" , "Capacity for rational thought" , "Sanity, the ability to think" .)