The most common and most likely answer to this crossword clue is the 3 letter word ELI
I've seen this crossword clue in The Guardian
E
L
I
More clues leading to the result ELI
- For Inquisition, the one priest
- Old priest the Spanish idolised originally
- Old priest: perhaps Miguel’s the one
- Old priest: the Spanish one
- Priest: the Spanish one
- Priest found in chapel indefinitely
- Priest taking books from George or Thomas Stearns?
- Priest regularly dropping bits of éclair
- Italian leader supporting the Spanish priest
- Folk above ground in Wells disowning old priest
Buddy explains!

The straight part
We know that "a priest" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case a priest = eli.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.