The most common and most likely answer to this crossword clue is the 8 plus 7 letter word CHEMICAL WARFARE
I've seen this crossword clue in The Guardian
C
H
E
M
I
C
A
L
W
A
R
F
A
R
E
Buddy explains!
The straight part
We know that "Form of struggle" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case form of struggle = chemical warfare.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.