Answer:
"An idea grew in my mind" is an idiom.
Explanation:
Idioms are phrases which create a specific and special meaning of a phrase.
So, the idiom "an idea grew in my mind" means "I came up with an idea" or "I have a new idea".
There is another frequent idiom with "mind" - "Broaden your mind", which means "to be more able to accept other people’s ideas and beliefs".
