An idiom is defined as - a group of words in a fixed order that has a particular meaning that is different from the meanings of each word on its own.
best part of (something)
to be almost all of something
We spent the best part of a day trying to fix the lamp.
List of common idioms, their meanings and examples of their usage in sentences
An idiom is a widely used saying or expression containing a figurative meaning that differs from the phrase's literal meaning.