State to carry back a wartime obstruction (7)
I believe the answer is:
caltrop
'a wartime obstruction' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'state to carry back' is the wordplay.
'state' becomes 'cal' (short for California).
'to' says to put letters next to each other.
'carry back' becomes 'trop' (I can't justify this - if you can you should give a lot more credence to this answer).
'cal'+'trop'='CALTROP'
Can you help me to learn more?
(Another definition for caltrop that I've seen is " Anti-cavalry weapon".)