Breaking down, how come? (4,2)
I believe the answer is:
show up
'come?' is the definition.
(thesaurus)
'breaking down how' is the wordplay.
'breaking' means one lot of letters goes inside another.
'down' becomes 'sup' (I can't explain this - if you can you should believe this answer much more).
'sup' enclosing 'how' is 'SHOW UP'.
Can you help me to learn more?
(Other definitions for show up that I've seen before include "Become visible" , "Arrive - put to shame" , "Make an appearance" , "Put in appearance or expose" , "Expose or keep appointment" .)