Determining what the meaning of part of a character '益' is
Outlier gives the following definition:

FORM
益 depicts a bowl (皿) with water () flowing out of it, representing the original meaning “abundant, more than enough.”

With a reference to page 147 of:

裘錫圭，1991《文字學概要(再版)》，臺北市：萬卷樓，2006年2月再版8刷。

It then continues:

COMPONENTS

In 益,   “flowing water” is a form component.
皿
In 益, 皿 “bowl” is a form component.

Original forms looked something like:

It goes to show that the entire thing was originally the meaning part.