The most common and most likely answer to this crossword clue is the 5 letter word KNOUT
I've seen this crossword clue in The Daily Telegraph
K
N
O
U
T
Buddy explains!

The straight part
We know that "Instrument of torture" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case instrument of torture = knout.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 KNOUT mean?
KNOUT noun- A leather scourge (multi-tail whip), in the severe version known as 'great knout' with metal weights on each tongue, notoriously used in imperial Russia.
- To flog or beat with a knout.