Substantial figure he had invested in former college and not returned (10)
I believe the answer is:
polyhedron
'substantial figure' is the definition.
'he had invested in former college and not returned' is the wordplay.
'he had' becomes 'hed'.
'invested in' means one lot of letters goes inside another.
'former college' becomes 'poly' (polytechnic).
'and not' becomes 'nor' (I've seen this before).
'returned' is a reversal indicator.
'nor' reversed gives 'ron'.
'poly'+'ron'='polyron'
'hed' placed into 'polyron' is 'POLYHEDRON'.
(Other definitions for polyhedron that I've seen before include "regular form?" , "Plane-faced solid" , "sugar cube perhaps" .)