I think there is something inherently wrong with the question.

There is a reason why modern dictionaries increasingly opt for "used to...", "used when...", "if you..., you...":

the matter [singular]
used (to ask) if somebody is upset, unhappy, etc. or if there is a problem
What's the matter? Is there something wrong?
(source)

Most of the time a word does not function in isolation but as a part of a whole.

It is actually more precise not to try to precisely equate a word or expression with another word or expression, but rather define the situation in which it is used, the purpose for which it is used, the function it has.

Assuming the above:

For what something/someone (truly) is
is used to say that you are able to perceive and understand the true nature of someone or something. (source)

In the same vein, I would say:

have something for something
is used to say that something can be imagined as something else or would be more aptly described as something else
I have a sack of potatoes for a belly.
In this day and age we have blockheads for influencers and gurus.
(source: me)

I think there is something inherently wrong with the question.

There is a reason why modern dictionaries increasingly opt for "used to...", "used when...", "if you..., you...":

the matter [singular]
used (to ask) if somebody is upset, unhappy, etc. or if there is a problem
What's the matter? Is there something wrong?
(source)

Most of the time a word does not function in isolation but as a part of a whole.

It is actually more precise not to try to precisely equate a word or expression with another word or expression, but rather define the situation in which it is used, the purpose for which it is used, the function it has.

Assuming the above:

For what something/someone (truly) is
is used to say that you are able to perceive and understand the true nature of someone or something. (source)

In the same vein, I would say:

have something for something
is used to say that something can be imagined as something else or would be more aptly described as something else
I have a sack of potatoes for a belly.
In this day and age we have blockheads for influencers and gurus.
(source: me)