I read the answer as Increment where the hint is the word "AFTER" indicating that the answer is "addition" after the action of changing L for R in inclement. I do agree it is a tough one and the counter argument has merit!
The way I read the clue is - cryptic indication: what would happen if we changed the R/L in a word meaning addition? Answer: we'd get a word meaning unfair". So I'm with the Ls.