The state Reg is in (6)
I believe the answer is:
oregon
'in' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.
'the state reg' is the wordplay.
'the state' becomes 'OR'.
'reg' becomes 'egon' (I can't explain this - if you can you should believe this answer much more).
'or'+'egon'='OREGON'
'is' is the link.
Can you help me to learn more?
(Other definitions for oregon that I've seen before include "America's Beaver State" , "State of north-west US" , "American Pacific state" , "Old trail led here" , "State near Washington" .)