Stuff up working with international inventor (7)
I believe the answer is:
marconi
'inventor' is the definition.
(I've seen this before)
'stuff up working with international' is the wordplay.
'stuff' becomes 'cram' (cramming is a kind of stuffing).
'up' says the letters should be written backwards (in down clue: letters go upwards).
'working' becomes 'on' (on can mean working or activated).
'with international' becomes 'i' (abbreviation eg in IMF. I am not sure about the 'with' bit.).
'cram' backwards is 'marc'.
'marc'+'on'+'i'='MARCONI'
Can you help me to learn more?
(Other definitions for marconi that I've seen before include "Italian physicist" , "Radio engineer" , "Pioneer of wireless" , "The father of radio" , "Developer of radio telegraphy" .)