reasoning

[mmasny]

Where's the accent in this word?

[euncu]

At the very beginning; reasoning (in IPA symbols; ˈriːzənɪŋ)

[mmasny]

I'm asking because of this: reasoning - definition of reasoning by the Free Online Dictionary, Thesaurus and Encyclopedia.
The second entry gives what you're saying. The first if I read it correctly says the secound syllable is stressed. What's the explanation?

[euncu]

No, the first one says the same. In IPA notation, the apostrophe is put before the stress. The one you mentioned puts the apostrophe after the stress. Since two different phonetic alphabets' notations are used on the same page, it's quite natural for one to be confused.