The most common and most likely answer to this crossword clue is the 6 plus 6 letter word STRIKE WEAPON
I've seen this crossword clue in The Daily Telegraph
S
T
R
I
K
E
W
E
A
P
O
N
Buddy explains!
The straight part
We know that "Club that’s used by militant trade unionists" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case club that’s used by militant trade unionists = strike weapon.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.