Girl bather in trouble (6)
I believe the answer is:
bertha
'girl' is the definition.
'bather in trouble' is the wordplay.
'in trouble' indicates anagramming the letters.
'bather' anagrammed gives 'BERTHA'.
(Other definitions for bertha that I've seen before include "Big girl" , "Big - - , WW1 gun" , "big woman?" , "Female name; Big -, World War one German howitzer" , "collar" .)