Correct spelling, explanation: complainant is a word constructed by the word-formation process known as suffixation, which means adding a suffix to a base word. In this case, similarly to other words using the -ant suffix, the base word is a verb. The suffix -ant has a meaning similar to a person who…, so using a verb is a logical choice. In this case, complain is the root of complainant.

Definition of complainant:
noun
1. in a legal context, a plaintiff; a person who sues
It turned out that the accusation was false, and the complainant lost the battle.
It’s not often that you see a mother as a complainant and a daughter as a defendant.

Incorrect spelling, explanation: complaintant seems like a logical spelling but is incorrect. If you wonder why, think of examples of similar words ending in a suffix -ant, such as assailant, created from assail, not assault. This is also quite smart because by this logic complainant means a person who complains, which fits the meaning well.

Incorrect spelling