The most common and most likely answer to this crossword clue is the 8 letter word MILITARY
I've seen this crossword clue in The Guardian
M
I
L
I
T
A
R
Y
More clues leading to the result MILITARY
- Forces new trial in my case and one is held
- Trial I organised in my army
- Soldiers on, breaching target over railroad
- Soldiers’ limit changed on a railway
- Perhaps, armed forces immorality... operating in absence of Medical Officer
- I landed amongst new army — the army!
- Landed on island, in custody of Catholic queen’s army
Buddy explains!

The straight part
We know that "Forces" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case forces = military.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 MILITARY mean?
MILITARY noun- Armed forces.
- Characteristic of members of the armed forces.
- Relating to armed forces such as the army, marines, navy and air force (often as distinguished from civilians or police forces).
- Relating to war.
- Relating to armies or ground forces.