Conservative's response first put on line (11)
I believe the answer is:
reactionary
'conservative's' is the definition.
(reactionary is a kind of conservative)
'response first put on line' is the wordplay.
'response' becomes 'reaction' (synonyms).
'first' becomes 'a' (first letter of the alphabet).
'put on' is a charade indicator (letters next to each other).
'line' becomes 'ry' (abbreviation for railway).
'reaction'+'a'+'ry'='REACTIONARY'
(Other definitions for reactionary that I've seen before include "Opposing progress or reform" , "Ultra-conservative" , "Diehard" , "No progressive" , "right-winger?" .)