The most common and most likely answer to this crossword clue is the 4 letter word POLY
I've seen this crossword clue in Independent
P
O
L
Y
Buddy explains!

The straight part
We know that "Old college" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case old college = poly.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 POLY mean?
POLY noun- Polytechnic.
- Polyethylene (polythene).
- Polyurethane
- Polygon.
- Polyester.
- Polyamory.
- A polyamorous person.
- To polymorph; to transform by magic.
- Polyamorous.
- A whitish woolly plant (Teucrium polium) of the family Labiatae, found throughout the Mediterranean.