The most common and most likely answer to this crossword clue is the 11 letter word NONBELIEVER
I've seen this crossword clue in The Times
N
O
N
B
E
L
I
E
V
E
R
Buddy explains!
The straight part
We know that "Infidel" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case infidel = nonbeliever.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 NONBELIEVER mean?
NONBELIEVER noun- A person who does not believe, especially regarding religion.