Ruin almost stops theologian, old priest (5)
I believe the answer is:
druid
'old priest' is the definition.
(I've seen this before)
'ruin almost stops theologian' is the wordplay.
'almost' means to remove the last letter (most of the word but not all of it).
'stops' means one lot of letters goes inside another.
'theologian' becomes 'DD' (Doctor of Divinity degree).
'ruin' with its final letter removed is 'rui'.
'rui' put within 'dd' is 'DRUID'.
(Other definitions for druid that I've seen before include "Member of Welsh Gorsedd; ancient priest" , "old magician?" , "Pre-Christian Celtic priest" , "Ancient Celtic priest" , "An Eisteddfod official" .)