Movement in Io as possibly captured by thousands (6)
I believe the answer is:
maoism
'movement' is the definition.
(Maoism is a political movement)
'io as possibly captured by thousands' is the wordplay.
'possibly' indicates anagramming the letters.
'captured by' is an insertion indicator.
'thousands' becomes 'mm' (Roman numerals).
'io'+'as'='ioas'
'ioas' is an anagram of 'aois'.
'aois' inserted into 'mm' is 'MAOISM'.
'in' is the link.
(Other definitions for maoism that I've seen before include "political theory" , "Chinese communist doctrine" .)