The most common and most likely answer to this crossword clue is the 4 letter word LOAN
I've seen this crossword clue in Financial Times
L
O
A
N
More clues leading to the result LOAN
- In Tiepolo, a notable advance
- Might this be secured without partner given mention?
- Advance unaccompanied, so to speak
- Dictator’s solitary sub
- See article in advance
- Advance without backup, say
- Look, a new mortgage?
- . Report single advance
- Say, a horse or cow, changing sides to advance
- Speaker's solitary advance
Buddy explains!

The straight part
We know that "sub" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case sub = loan.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 LOAN mean?
LOAN noun- An act or instance of lending, an act or instance of granting something for temporary use.
- A sum of money or other property that a natural or legal person borrows from another with the condition that it be returned or repaid over time or at a later date (sometimes with interest).
- The contract and array of legal or ethical obligations surrounding a loan.
- The permission to borrow any item.
- To lend (something) to (someone).
- A lonnen.