Conservative voting i.e. altered related to reasoning (9)
I believe the answer is:
cognitive
'related to reasoning' is the definition.
(relation to thought or cognition)
'conservative voting i e altered' is the wordplay.
'conservative' becomes 'C' (UK political abbreviation).
'altered' indicates anagramming the letters.
'voting'+'i'+'e'='votingie'
'votingie' with letters rearranged gives 'ognitive'.
'c'+'ognitive'='COGNITIVE'
(Other definitions for cognitive that I've seen before include "Knowing" , "perceptive thinking" .)