The most common and most likely answer to this crossword clue is the 9 letter word POSTULANT
I've seen this crossword clue in The Daily Telegraph
P
O
S
T
U
L
A
N
T
Buddy explains!

The straight part
We know that "candidate" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case candidate = postulant.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.
What does POSTULANT mean?
POSTULANT noun- A person seeking admission to a religious order
- A person who submits a petition for something; a petitioner.