Dave is back in an upset state (6)
I believe the answer is:
nevada
'state' is the definition.
(Nevada is an example)
'dave is back in an upset' is the wordplay.
'is back' shows that the letters should be reversed in order.
'in' means one lot of letters goes inside another.
'upset' says the letters should be written in reverse.
'dave' backwards is 'evad'.
'an' written backwards gives 'na'.
'evad' inserted into 'na' is 'NEVADA'.
(Other definitions for nevada that I've seen before include "US state, capital Carson City" , "Western US state" , "US state, NV" , "Curiously, Dan and Eva are in the US" , "part of US" .)