She was a big thing in World War I (6)
I believe the answer is:
bertha
'she' is the definition.
'war i' is the wordplay.
'war' becomes 'berth' (I can't justify this - if you can you should believe this answer much more).
'i' becomes 'a'.
'berth'+'a'='BERTHA'
'was a big thing in world' is the link.
This may not be right. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for bertha that I've seen before include "Female name; Big -, World War one German howitzer" , "Breath (anag) - girl's name" , "big woman?" , "Big ..., heavy German howitzer in World War 1" , "Big girl" .)