Memory returned when back in girls' bog (6)
I believe the answer is:
morass
'bog' is the definition.
(I know that morass is a type of bog)
'memory returned when back in girls'' is the wordplay.
'memory' becomes 'ROM' (Read-Only Memory on a computer).
'returned' shows that the letters should be reversed in order.
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'back in' says to take the final letters (the 'back' of the word).
The last letter of 'girls' is 's'.
'rom' in reverse letter order is 'mor'.
'mor'+'as'+'s'='MORASS'
(Other definitions for morass that I've seen before include "Swampy, low-lying land" , "Muddled situation" , "Bog - situation hard to get out of" , "dangerous situation" , "Bog or swamp" .)