Minimum bill permitted in Northern Ireland (5)
I believe the answer is:
nadir
'minimum' is the definition.
(thesaurus)
'bill permitted in northern ireland' is the wordplay.
I cannot really see how this works, but
'bill' could be 'ad' (advertisement stuck on wall) and 'ad' is located in the answer.
'northern' could be 'n' (abbreviation e.g. N. Ireland) and 'n' is located in the answer.
'ireland' could be 'ir' (abbreviation for Ireland) and 'ir' is located in the answer.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for nadir that I've seen before include "deep despair" , "Dinar (anag.)" , "most unsuccessful point" , "least successful point" , "You can't sink lower!" .)