The most common and most likely answer to this crossword clue is the 9 letter word UNHARNESS
I've seen this crossword clue in The Times
U
N
H
A
R
N
E
S
S
More clues leading to the result UNHARNESS
Buddy explains!

The straight part
We know that "take armour off" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case take armour off = unharness.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 UNHARNESS mean?
UNHARNESS verb- To remove the harness from a horse etc.
- (by extension) to liberate