Ruddy turned on about pinching one's buttocks (8)
I believe the answer is:
derriere
'buttocks' is the definition.
(synonyms)
'ruddy turned on about pinching one's' is the wordplay.
I cannot quite understand how this works, but
'on' could be 're' ('on' can be mean 'with reference to') and 're' is present in the answer.
'one' could be 'i' (Roman numeral) and 'i' is present in the answer.
'about' could be 're' ('re' means regarding or about) and 're' is found in the answer.
The remaining letters 'der' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for derriere that I've seen before include "One's backside in France" , "Seat" , "A French backside or buttocks" , "(French?) behind" , "Fourteen down's [FRENCHMAN] behind" .)