This is a charade - multiple parts of the clue combine to spell out the answer when joined together.
'retrospectively' indicates reversal - take 'suing' (legal action) backwards to get SUINEG, but you need to find the right combination with 'equation's origin' (first letter of 'equation' = E) and how they fit together as GE + NIUS.
Answer: GENIUS | 'retrospectively' signals reversal - 'suing' (legal action) backwards gives GNUIS, combined with 'equation's origin' (E from equation) to form GE-NIUS