What does the cryptic crossword clue "Idiot holding back star pupil? On the contrary (4)" mean?

The definition is "Idiot". The wordplay uses a reversal. Hidden reversed in "idiot holding back sTAR Pupil? on the contrary" The answer is PRAT.

Idiot holding back star pupil? On the contrary (4)

Guardian Cryptic #28874 | 18 Across | 2022-09-28 flower