Sister checks it out for leader's ambassador (6)
I believe the answer is:
nuncio
'ambassador' is the definition.
(thesaurus)
'sister checks it out for leader's' is the wordplay.
I cannot really see how this works, but
'sister' could be 'nun' (I've seen this before) and 'nun' is located in the answer.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for nuncio that I've seen before include "See man" , "Diplomatic representative of the Pope" , "Papal emissary" , "Vatican delegate" , "Papal ambassador" .)