The most common and most likely answer to this crossword clue is the 7 letter word KOPECKS
I've seen this crossword clue in The Guardian
K
O
P
E
C
K
S
Buddy explains!
The straight part
We know that "capital of Siberia, say?" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case capital of siberia, say? = kopecks.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 KOPECKS mean?
KOPECKS noun- A Russian monetary unit equal to one hundredth of a ruble.
- A kopiyka: a Ukrainian monetary unit equal to one hundredth of a hryvnia.