If at all

Neither reads correctly. Neither is a complete sentence or seems to represent a complete thought. More information about the context is needed. It is not clear whether you intend them to be complete sentences.

It is a footnote in my essay. A footnote to:

One of the main topics of metaphor research is concerned with how ‘literal meaning’#_ftn1plays a role in the understanding of metaphor.

#_ftnref1’Literal meaning’ whatever it is or in whatever terms it is defined by researchers, i

Sorry, I can't understand what you are trying to explain. If you remove the quotes from "literal meaning" and delete the footnote then the sentence makes sense. I don't see what aspect of "literal meaning" requires further explanation.

I am trying to say that although researchers talk about the role of literal meaning in metaphor understanding, they do not take the trouble to define literal meaning itself. So, I have these alternatives:

1. ’Literal meaning’ whatever it is or in whatever terms it is defined by researchers, if at all.
2. ’Literal meaning’, in whatever terms it is defined, if at all, by the researchers

Neither of those reads well. It is unsurprising that researchers do not define "literal meaning" because its meaning is self-evident. I think your footnote is unnecessary, or, if it is necessary, more explanation to the reader about why it is necessary is required. For example:

"Literal meaning" is subject to various interpretations [give an idea of what these are, as it is not obvious