Most inactive and poorly during Second International (8)
I believe the answer is:
stillest
'most inactive' is the definition.
('still' can be a synonym of 'inactive')
'poorly during second international' is the wordplay.
'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' (I've seen this before).
's'+'test'='stest'
'ill' put into 'stest' is 'STILLEST'.
'and' is the link.
(Other definitions for stillest that I've seen before include "Least disruptive" , "Most calm" , "least enterprising" .)