Came to a conclusion (5)
I believe the answer is:
ended
'came to a conclusion' is the definition.
(I've seen this in another clue)
'came to a conclusion' is the wordplay.
'came' becomes 'ed' (I can't justify this - if you can you should believe this answer much more).
'to a' means one lot of letters go next to another (I've seen this in other clues).
'conclusion' becomes 'end' (something's conclusion is its end).
'ed' put after 'end' is 'ENDED'.
Can you help me to learn more?
(Other definitions for ended that I've seen before include "Broke off; finished" , "Concluded" , "Brought to a finish" , "Done" , "Stopped" .)