As recommended by Mauro ALLEGRANZA, looking at some different available translations sheds light on what might be the intended meaning here.
In James Adam's translation, rather than 'informer', the word 'pettifogger' is used (emphasis mine):
That is because you argue like a pettifogger, Socrates. Why, to take
the nearest example, do you call one who is mistaken about the sick a
physician in respect of his mistake or one who goes wrong in a
calculation a calculator when he goes wrong and in respect of this
error? Yet that is what we say literally—we say that the physician
erred and the calculator and the schoolmaster. But the truth, I take
it, is, that each of these in so far as he is that which we entitle
him never errs; so that, speaking precisely, since you are such a
stickler for precision, no craftsman errs. For it is when his
knowledge abandons him that he who goes wrong goes wrong—when he is
not a craftsman. So that no craftsman, wise man, or ruler makes a
mistake then when he is a ruler, though everybody would use the
expression that the physician made a mistake and the ruler erred. It
is in this loose way of speaking, then, that you must take the answer
I gave you a little while ago. But the most precise statement is that
other, that the ruler in so far forth as ruler does not err, and not
erring he enacts what is best for himself, and this the subject must
do, so that, even as I meant from the start, I say the just is to do
what is for the advantage of the stronger.
Looking at the dialogue above which follows, a suitable interpretation of 'pettifogger' here would be the second definition of Merriam Webster:
one given to quibbling over trifles
Or Google's definition of 'pettifogging':
placing undue emphasis on petty details; petty or trivial.
Later, in Adam's translation, rather than 'to cheat', the word 'pettifogger' is reused:
“Why, do you suppose,” I said, “that I am so mad to try to try to
beard a lion and try the pettifogger on Thrasymachus?” “You did try
it just now,” he said, “paltry fellow though you be.” “Something too
much of this sort of thing,” said I.
It thus seems that Thrasymachus claims that Socrates 'cheated' earlier by detracting from his argument by intentionally focusing on (misinterpreting?) minor details.