Context:
>>What is No Such Agency - Q group?
>>Who has clearance to full picture?
>>Important.
>>SIS is good.
>>+++Adm R+++
>>What agency is at war w/ Clowns In America?
Likely US military related given everything Q posted up to that point, stating that MI was being used by POTUS. This line indicates that SIS has been secretly reformed.
Signal Intelligence Service
www.nsa. gov/about/cryptologic-heritage/center-cryptologic-history/pearl-harbor-review/sis.shtml
That is how I interpreted it at the time, I think it is still a good interpretation. MI6 (non-US intelligence) wasn't implied anywhere.