Have fights, splitting theologians vertically (9)
I believe the answer is:
downwards
'vertically' is the definition.
'have fights splitting theologians' is the wordplay.
'have' becomes 'own' (synonyms).
'fights' becomes 'war'.
'splitting' is an insertion indicator.
'theologians' becomes 'dds' (DD is short for Doctor of Divinity).
'own'+'war'='ownwar'
'ownwar' placed within 'dds' is 'DOWNWARDS'.
(Other definitions for downwards that I've seen before include "Towards a lower point" , "as seen in feather-bedded hospitals?" , "Direction of the pull of gravity" .)