Wood one initially cut for reason (5)
I believe the answer is:
logic
'for reason' is the definition.
(logic is using reason)
'wood one initially cut' is the wordplay.
'wood' becomes 'log' (a log is a piece of wood).
'one' becomes 'I' (Roman numeral).
'initially' suggests taking the first letters.
The initial letter of 'cut' is 'c'.
'log'+'i'+'c'='LOGIC'
(Other definitions for logic that I've seen before include "subject with 14 [ARGUMENTS]" , "method of reasoning" , "Reasoned argument" , "Rational thinking" , "Aristotelian theories" .)