Where men fought in an era that was backward (5)
I believe the answer is:
arena
'where men fought' is the definition.
The answer is a location as well as being a singular noun. This is suggested by the definition.
'in an era that was backward' is the wordplay.
'in' is an insertion indicator.
'an' becomes 'a'.
'that was' becomes 'n' (I can't explain this - if you can you should believe this answer much more).
'backward' says the letters should be written backwards.
'a'+'era'='aera'
'aera' enclosing 'n' is 'anera'.
'anera' reversed gives 'ARENA'.
Can you help me to learn more?
(Other definitions for arena that I've seen before include "Area of combat" , "Place for public contests" , "Scene of action" , "Action space in centre of amphitheatre" , "Enclosure for sports etc." .)