Theory oddly lacking conclusion (8)
I believe the answer is:
terminus
'conclusion' is the definition.
('terminus' can be a synonym of 'conclusion')
'theory oddly lacking' is the wordplay.
'oddly' indicates alternate letters.
'lacking' becomes 'minus' (I've seen this before).
The alternating letters of 'theory' are 'ter'.
'ter'+'minus'='TERMINUS'
(Other definitions for terminus that I've seen before include "Limit" , "Where something ends" , "End of a railway or bus route" , "Victoria, perhaps" , "Destination point" .)