Entering myself, in a terrible state (5)
I believe the answer is:
maine
'state' is the definition.
(US state)
'entering myself in a terrible' is the wordplay.
'entering' means one lot of letters goes inside another.
'myself' becomes 'me'.
'terrible' indicates anagramming the letters.
'in'+'a'='ina'
'ina' with letters rearranged gives 'ain'.
'me' going around 'ain' is 'MAINE'.
(Other definitions for maine that I've seen before include "N. US Atlantic state" , "New England state" , "NE US state" , "somewhere in America" , "Part of America" .)