Girl's ace on bed! (6)
I believe the answer is:
bertha
'girl's' is the definition.
'ace on bed' is the wordplay.
'ace' becomes 'a'.
'on' means one lot of letters go next to another.
'bed' becomes 'berth' (berth is a kind of bed).
'a' put after 'berth' is 'BERTHA'.
(Other definitions for bertha that I've seen before include "Female name; Big -, World War one German howitzer" , "43-ton German howitzer, Big _____" , "Breath (anag) - girl's name" , "Girl's name - big German WWI gun" , "The bar (anag.)" .)