Door collapsed by your old lady (7)
I believe the answer is:
dorothy
'lady' is the definition.
'door collapsed by your old' is the wordplay.
'collapsed' indicates an anagram.
'by' means one lot of letters go next to another.
'your old' becomes 'thy'.
'door' anagrammed gives 'doro'.
'doro'+'thy'='DOROTHY'
(Other definitions for dorothy that I've seen before include "Name of the young heroine in 'The Wizard of Oz'" , "Woman" , "one of 5d's troupe" , "Wizard of Oz child" , "'Wizard of Oz' girl" .)