On reflection, did wrong pinching one monarch's bottom (8)
I believe the answer is:
derriere
'bottom' is the definition.
(derriere can mean someone's bottom)
'on reflection did wrong pinching one monarch's' is the wordplay.
'on reflection' says the letters should be written backwards.
'did wrong' becomes 'erred' (err can mean to sin or do wrong).
'pinching' means one lot of letters goes inside another (taking inside).
'one' becomes 'i' (Roman numeral).
'monarch' becomes 'er' (abbreviation for Elizabeth Regina, British Queen).
'erred' written backwards gives 'derre'.
'i'+'er'='ier'
'derre' enclosing 'ier' is 'DERRIERE'.
(Other definitions for derriere that I've seen before include "A French backside or buttocks" , "(French?) behind" , "One's backside in France" , "Seat" , "Fourteen down's [FRENCHMAN] behind" .)