Million invested in vaccine support (4)
I believe the answer is:
jamb
'support' is the definition.
(jamb is a kind of support)
'million invested in vaccine' is the wordplay.
'million' becomes 'm' (abbreviation).
'invested in' indicates putting letters inside.
'vaccine' becomes 'jab' (I can't explain this - if you can you should believe this answer much more).
'm' inserted within 'jab' is 'JAMB'.
Can you help me to learn more?
(Other definitions for jamb that I've seen before include "Upright sidepiece" , "Vertical side of a doorway" , "Side post of a doorway" , "part of doorway?" , "bit on the side?" .)