The most common and most likely answer to this crossword clue is the 5 letter word HALER
H
A
L
E
R
Buddy explains!

The straight part
We know that "Unit of currency/increasingly robust" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case unit of currency/increasingly robust = haler.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 HALER mean?
HALER adjective- Sound, entire, healthy; robust, not impaired.
- A German coin equivalent to half a pfennig, later used widely as a small coin in Central Europe and the German Empire.
- A subdivision of the Czech, Slovakian and Czechoslovakian koruna. 100 hellers make up one koruna.