Actually, that may be the problem with many purported answers to your question.

Two two most often awarded the “longest one-word palindrome” award (each with 12 letters) are:

a) TATTARRATTAT–longest one listed in the Oxford English Dictionary. This word was invented by James Joyce to indicate the sound of someone knocking on the door.

b) DETARTRATED – a chemical term (though a rather contrived one) which the Guinness book accepts as the longest

Close behind are:

d) KINNIKINNIK (11 letters) – “a preparation made from dried leaves, bark, and sometimes tobacco and smoked especially by certain Native American peoples”. This one, at least, was not invented for this purpose (I don’t think!)

d) REDIVIDER (10 letters) — this appears to be the longest “ordinary” word (one not invented to BE long, ‘native English’, and one you might actually find in use!)

Edit:

All words are made up at some point, but I did not make this one up. It was actually a question on the Millionaire game show!

hannah?

hope this helps.