Corporation has invested large surplus (4)
I believe the answer is:
glut
'surplus' is the definition.
(glut can mean a surplus or surfeit)
'corporation has invested large' is the wordplay.
'corporation' becomes 'gut' (both can humorously mean belly fat).
'has invested' indicates putting letters inside.
'large' becomes 'L' (eg in clothes sizes).
'gut' going around 'l' is 'GLUT'.
(Other definitions for glut that I've seen before include "Excess, surfeit" , "stuff" , "An excessive supply, maybe on the market" , "An excessive supply or surfeit" , "Satiate" .)