Girl changes her bat (6)
I believe the answer is:
bertha
'girl' is the definition.
'changes her bat' is the wordplay.
'changes' indicates an anagram.
'her'+'bat'='herbat'
'herbat' anagrammed gives 'BERTHA'.
(Other definitions for bertha that I've seen before include "big woman?" , "Big ..., heavy German howitzer in World War 1" , "Big - - , WW1 gun" , "collar" , "Girl's name - big German WWI gun" .)