The most common and most likely answer to this crossword clue is the 6 plus 6 letter word SECRET POLICE
S
E
C
R
E
T
P
O
L
I
C
E
More clues leading to the result SECRET POLICE
Buddy explains!

The straight part
We know that "shadowy force" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case shadowy force = secret police.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.