Beside the bed is a girl (6)
I believe the answer is:
bertha
'girl' is the definition.
'beside the bed is a' is the wordplay.
'beside' says to put letters next to each other.
'the bed' becomes 'berth' (berth is a kind of bed).
'berth' put next to 'a' is 'BERTHA'.
(Other definitions for bertha that I've seen before include "big woman?" , "collar" , "Big ..., heavy German howitzer in World War 1" , "Big - - , WW1 gun" , "Girl's name - big German WWI gun" .)