The most common and most likely answer to this crossword clue is the 10 letter word PAWNBROKER
I've seen this crossword clue in The Guardian
P
A
W
N
B
R
O
K
E
R
More clues leading to the result PAWNBROKER
Buddy explains!

The straight part
We know that "Uncle" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case uncle = pawnbroker.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 PAWNBROKER mean?
PAWNBROKER noun- A person who makes monetary loans at interest, taking personal property as security – which may be sold if not redeemed.