One taking part in the arms race? (9)
I believe the answer is:
gunrunner
'one taking part' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'in the arms race?' is the wordplay.
'in the' means one lot of letters goes inside another.
'arms' becomes 'gunner' (I can't justify this - if you can you should give a lot more credence to this answer).
'race?' becomes 'run' (synonyms).
'gunner' enclosing 'run' is 'GUNRUNNER'.
Can you help me to learn more?
(Other definitions for gunrunner that I've seen before include "Illegal arms dealer" , "one secretly moves pieces?" , "one who moves arms impermissibly" , "Arms smuggler" .)