Theologian in the distance is halfway (6)
I believe the answer is:
middle
'halfway' is the definition.
('middle' can be a synonym of 'halfway')
'theologian in the distance' is the wordplay.
'theologian' becomes 'dd' (Doctor of Divinity degree).
'in the' means one lot of letters goes inside another.
'distance' becomes 'mile'.
'dd' placed within 'mile' is 'MIDDLE'.
'is' acts as a link.
(Other definitions for middle that I've seen before include "One of 21 [HEARTS]" , "Heart" , "Group of clubs; distance" , "Centre or core" , "Equally distant from the extremes" .)