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 cannot see how one could define the other.
'again and went back in' is the wordplay.
'again' becomes 're' (prefix meaning 'again').
'and' means one lot of letters go next to another.
'went back in' becomes 'entered' (I am not sure about this - if you are sure you should believe this answer much more).
'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".)