Curse company that's invested in sudden increase (7)
I believe the answer is:
scourge
'curse' is the definition.
(I know that curse can be written as scourge)
'company that's invested in sudden increase' is the wordplay.
'company' becomes 'co' (abbreviation).
'that's invested in' indicates putting letters inside.
'sudden increase' becomes 'surge' (a surge is a sudden increase in something).
'co' inserted within 'surge' is 'SCOURGE'.
(Other definitions for scourge that I've seen before include "One causing great suffering; whip" , "Curse; whip" , "Punish severely" , "Afflict" , "Bane, curse" .)