False flat
A word or phrase becomes another when some grammatical change is inappropriately applied. For example, a false plural: inter, interim (à la seraph, seraphim or cherub, cherubim).
The solution: PRIMAL = butter, FINAL = buttress (à la waiter and waitress.)
Some other examples will show you how many possibilities there are. The false antecedent: sent, present; or sting, foresting. The false comparative: limb, limber. The false gerund: shill, shilling. The false negative: plus, nonplus; or gone, Antigone. The false past tense: worst, worsted; or live, lave (analogy with give and gave). The false reiterative: treat, retreat.
False flats based on very obscure derivations are fairly common. In such cases, to be fair to solvers, at least one word in the model should appear in the verse, or at least be very clearly clued. For example, a “false adherent” on instar and monster included both paradigm words (“Islam”, “Moslem”) in the verse. Another example is the false plural: shader, Shadrach (“knaidel” appeared in the verse; its plural is “knaidlach”). Or the false opposite: census, SASEs (based on the opposition “centum/satem”, explained in online MW; the word “centum” appeared in the verse, as did an explanation of the opposition.)