Used keys again and went back in (2-7)
I believe the answer is:
re-entered
'used keys' is the definition.
Although both the answer and definition are past participle verbs, I can't understand how they can define each other.
'again and went back in' is the wordplay.
'again' becomes 're' (prefix meaning 'again').
'and' says to put letters next to each other.
'went back in' becomes 'entered' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
're'+'entered'='RE-ENTERED'
Can you help me to learn more?
(Another definition for re-entered that I've seen is " Returned into the earth's atmosphere".)