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 really understand how this works, but
'on' could be 're' ('on' can be mean 'with reference to') and 're' is located in the answer.
'one' could be 'i' (Roman numeral) and 'i' is located in the answer.
'about' could be 're' (regarding) and 're' is located 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" .)