The most common and most likely answer to this crossword clue is the 8 letter word ORDNANCE
I've seen this crossword clue in The Times
O
R
D
N
A
N
C
E
More clues leading to the result ORDNANCE
- Arms injunction I ignored
- Big guns knight invited into ranker’s function
- Artillery hit car end-on
- Weaponry law not keeping current
- Big guns planning to sack international
- . We see no dancer waving arms
- Alternative nob inserted in ball – old boy going great guns!
- Directive almost involving Reagan trimmed down the big guns
- Soldiers cavort around end of town, displaying military equipment
- Arms whirling on dancer
Buddy explains!

The straight part
We know that "Big guns" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case big guns = ordnance.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 ORDNANCE mean?
ORDNANCE noun- Military equipment, especially weapons and ammunition.
- Artillery.