Island nearly is so invaded by millions (7)
I believe the answer is:
isthmus
'island nearly' is the definition.
(a narrow strip of land)
'is so invaded by millions' is the wordplay.
'so' becomes 'thus' (synonyms).
'invaded by' indicates putting letters inside.
'millions' becomes 'm' (abbreviation).
'thus' enclosing 'm' is 'thmus'.
'is'+'thmus'='ISTHMUS'
(Other definitions for isthmus that I've seen before include "Narrow strip of land joining two larger entities" , "Narrow strip of land connecting two large masses" , "Narrow strip of land with water on two sides" , "This sum for a narrow piece of land" , "Such a narrow area" .)