One taking part (5)
I believe the answer is:
actor
'one taking part' is the definition.
(I've seen this before)
'part' is the wordplay.
I cannot really understand how this works, but
'part' could be 'act' (I have seen 'Part of play ' mean 'act' so perhaps 'part' could also mean 'act') and 'act' is located in the answer.
The remaining letters 'or' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for actor that I've seen before include "Thespian, stage performer" , "ham" , "Luvvy" , "Player - performer" , "Gielgud, say" .)