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 don't see how they can define each other.
'again and went back in' is the wordplay.
'again' becomes 're' (prefix meaning 'again').
'and' is a charade indicator (letters next to each other).
'went back in' becomes 'entered' (I can't explain this - if you can 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".)