Great old plane maybe carrying millions (7)
I believe the answer is:
extreme
'great' is the definition.
(synonyms)
'old plane maybe carrying millions' is the wordplay.
'old' becomes 'ex'.
'plane maybe' becomes 'tree' (plane is a type of tree).
'carrying' is an insertion indicator.
'millions' becomes 'm' (abbreviation).
'tree' going around 'm' is 'treme'.
'ex'+'treme'='EXTREME'
(Other definitions for extreme that I've seen before include "very strict" , "Intense; furthest away" , "Farthest - drastic" , "furthest from the centre" , "over the top" .)