The most common and most likely answer to this crossword clue is the 6 letter word KRONER
I've seen this crossword clue in Financial Times
K
R
O
N
E
R
Buddy explains!

The straight part
We know that "foreign currency" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case foreign currency = kroner.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 KRONER mean?
KRONER noun- The currency of Iceland, Denmark (including Greenland and the Faroe Islands) and Norway, divided into 100 øre, except in Iceland where 1 króna = 100 aurar.