Plant opened by appropriate tool, entered again (7)

'entered again' is the definition.
Both the answer and definition are past participle verbs.
Perhaps they are linked in a way I don't understand?

'plant opened by appropriate tool' is the wordplay.
'plant' becomes 'reed' (reed is a kind of plant).
'opened by' indicates putting letters inside (one word creates an opening in another word).
'appropriate tool' becomes 'key' (I can't explain this - if you can you should give a lot more credence to this answer).
'reed' enclosing 'key' is 'REKEYED'.

Can you help me to learn more?