Falsify getting infiltrated by unknown country (6)
I believe the answer is:
belize
'country' is the definition.
(I know that Belize is a country)
'falsify getting infiltrated by unknown' is the wordplay.
'falsify' becomes 'belie' (I've seen this in another clue).
'getting infiltrated by' indicates putting letters inside.
'unknown' becomes 'z' (algebra).
'belie' placed around 'z' is 'BELIZE'.
(Other definitions for belize that I've seen before include "Central American republic" , "Central American country, formerly British Honduras" , "Central American state" , "Commonwealth state" , "Country formerly called British Honduras" .)