Military alliance is not a problem (4)
I believe the answer is:
nato
'military alliance' is the definition.
(I've seen this before)
'not a problem' is the wordplay.
'problem' is an anagram indicator.
'not'+'a'='nota'
'nota' anagrammed gives 'NATO'.
'is' acts as a link.
(Other definitions for nato that I've seen before include "US/Europe alliance" , "Western mil. alliance" , "Military alliance, formed 1949" , "Atlantic alliance (abbr.)" , "International organisation for collective security" .)