Inflammation I insist is circulating outside United States (9)
I believe the answer is:
sinusitis
'inflammation' is the definition.
(sinusitis is a kind of inflammation)
'i insist is circulating outside united states' is the wordplay.
'is circulating' is an anagram indicator.
'outside' means one lot of letters goes inside another.
'united states' becomes 'us' (abbreviation for the United States).
'i'+'insist'='iinsist'
'iinsist' anagrammed gives 'sinitis'.
'sinitis' going around 'us' is 'SINUSITIS'.
(Other definitions for sinusitis that I've seen before include "nasal problem" , "Inflammation of head cavities" , "Inflammation in the skull" , "Inflammation of nasal cavities" , "Nasal membrane inflammation" .)