Arkansas and Idaho are very dry (4)
I believe the answer is:
arid
'very dry' is the definition.
(I've seen this before)
'arkansas and idaho' is the wordplay.
'arkansas' becomes 'ar' (I've seen this in other clues).
'and' means one lot of letters go next to another.
'idaho' becomes 'ID' (abbreviation for the US state).
'ar'+'id'='ARID'
'are' is the link.
(Other definitions for arid that I've seen before include "Parched with heat" , "Extremely dry" , "Very dry, having little rain" , "Without water" , "Dry and withered" .)