No particular one key part (3)
I believe the answer is:
any
'no particular' is the definition.
Both the answer and definition are adjectives. Maybe there's a link between them I don't understand?
'one key part' is the wordplay.
'one' becomes 'a'.
'key part' becomes 'ny' (I can't justify this - if you can you should give a lot more credence to this answer).
'a'+'ny'='ANY'
Can you help me to learn more?
(Other definitions for any that I've seen before include "A lot < ? Not a lot < -- some" , "'Whatever'" , "One, some, every or all without specification" , "Long-running Radio 4 discussion show" , "One, no matter which" .)