Most inactive and poorly during Second International (8)
I believe the answer is:
Here is my best explanation:
'most inactive' is the definition.
('still' can be a synonym of 'inactive')
'poorly during second international' is the subsidiary indication.
'poorly' becomes 'ill' ('ill' can be a synonym of 'poorly').
'during' means one lot of letters goes inside another.
'second' becomes 's' (common abbreviation).
'international' becomes 'test'.
'ill' put within 'stest' is 'stillest'.
'and' is the link.
(Another definition for stillest that I've seen is "Most calm".)