Where, in the first place, everyone did everything naked! (4)
I believe the answer is:
eden
'where in the first place everyone did everything naked' is the definition.
The answer is a location as well as being a singular noun. This is suggested by the definition.
'where in the first place' is the wordplay.
'where in' becomes 'e' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'the first' means one lot of letters go next to another.
'place' becomes 'den' (den is a kind of place).
'e'+'den'='EDEN'
(Other definitions for eden that I've seen before include "Delightful place" , "PM 1955-56" , "Idyllic garden" , "Place of complete bliss" , "The near-perfect garden" .)