At work, that man's enthralled by right-winger on paper (2,6)
I believe the answer is:
in theory
'on paper' is the definition.
(synonyms)
'at work that man's enthralled by right-winger' is the wordplay.
'at work' becomes 'in' (I've seen this in other clues).
'that man' becomes 'he' ('he' refers to a man).
'enthralled by' indicates putting letters inside ('enthral' can mean 'capture').
'right-winger' becomes 'Tory' (Tory is an example).
'he' put into 'tory' is 'theory'.
'in'+'theory'='IN THEORY'
(Other definitions for in theory that I've seen before include "On paper" , "according to the assumed facts" , "In principal" , "Ideally" .)