One taking part in an emergency (10)
I believe the answer is:
understudy
'one taking part in an emergency' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'one' is the wordplay.
I cannot really see how this works, but
'one' could be 'un' (northern English dialect form of 'one') and 'un' is located in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for understudy that I've seen before include "Replacement" , "Stage stand-in" , "reserve?" , "emergency role" , "One prepared to take over from leading actor" .)