Cryptic crossword clue
Substantial figure he had invested in former college and not returned 10

The most common and most likely answer to this crossword clue is the 10 letter word POLYHEDRON

I've seen this crossword clue in Independent
P
O
L
Y
H
E
D
R
O
N

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The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.
This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.

What does POLYHEDRON mean?

POLYHEDRON noun
  1. A solid figure with many flat faces and straight edges.
  2. A polyscope, or multiplying glass.

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