The most common and most likely answer to this crossword clue is the 6 letter word SUTLER
I've seen this crossword clue in Financial Times
S
U
T
L
E
R
More clues leading to the result SUTLER
Buddy explains!

The straight part
We know that "Old army supplier" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case old army supplier = sutler.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 SUTLER mean?
SUTLER noun- A person who follows an army, selling provisions.