They're written in eastern and southern states (6)
I believe the answer is:
essays
'they're written' is the definition.
The definition suggests an adverb but the answer is not.
'eastern and southern states' is the wordplay.
'eastern' becomes 'e'.
'and' means one lot of letters go next to another.
'southern' becomes 's' (abbreviation).
'states' becomes 'says' ('say' can be a synonym of 'state').
'e'+'s'+'says'='ESSAYS'
'in' acts as a link.
Can you help me to learn more?
(Other definitions for essays that I've seen before include "literary work" , "Short pieces of writing on a particular subject" , "Disquisitions" , "texts" , "Short formal pieces of writing" .)