Challenged a heartless rule adopted by theologian (5)
I believe the answer is:
dared
'challenged' is the definition.
(daring is a kind of challenging)
'a heartless rule adopted by theologian' is the wordplay.
'heartless' says to hollow out the word (remove centre letters).
'adopted by' is an insertion indicator (I've seen this in other clues).
'theologian' becomes 'dd' (Doctor of Divinity degree).
'rule' with its centre removed is 're'.
'a'+'re'='are'
'are' going inside 'dd' is 'DARED'.
(Other definitions for dared that I've seen before include "Challenged - was audacious" , "Was brave enough" , "Acted with courage - or dread?" , "Risked" , "Ventured" .)