An idiom is a phrase that has a figurative meaning that is different from the literal meaning of the individual words. For example, "kick the bucket" means to die, even though it doesn't literally refer to kicking a bucket.
"In a" is just a grammatical construction, and it doesn't have a separate figurative meaning.
Examples:
* "In a hurry" is a common phrase, but it doesn't have a figurative meaning beyond the literal meaning of "in" and "hurry."
* "In a nutshell" is a common phrase, but it doesn't have a figurative meaning beyond the literal meaning of "in" and "nutshell."
Let me know if you have any other phrases you'd like to check!