Able to enter Nato, regularly (9)
I believe the answer is:
competent
'able' is the definition.
(similar in meaning)
'enter nato regularly' is the wordplay.
'enter' becomes 'compete' (I can't justify this - if you can you should give a lot more credence to this answer).
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
The alternating letters of 'nato' are 'nt'.
'compete'+'nt'='COMPETENT'
'to' acts as a link.
Can you help me to learn more?
(Other definitions for competent that I've seen before include "Able (to do something)" , "Expert" , "Qualified" , "Efficient, capable" , "Teen drink" .)