Make a note of one constant reason (5)
I believe the answer is:
logic
'reason' is the definition.
(I've seen this before)
'make a note of one constant' is the wordplay.
'make a note' becomes 'log'.
'of one' becomes 'i' (I've seen this in other clues).
'constant' becomes 'c' (mathematical abbreviation).
'log'+'i'+'c'='LOGIC'
(Other definitions for logic that I've seen before include "4-ing" , "Science of correct reasoning" , "Strict reasoning" , "Rigorous reasoning" , "Rational thinking" .)