Outsiders from Marseille left German bread for French mathematician (10)
I believe the answer is:
mandelbrot
'outsiders from marseille' is the definition.
I know nothing about this answer so I cannot judge whether it can be defined by this definition.
'left german bread for french mathematician' is the wordplay.
'left' becomes 'l' (common abbreviation).
'german bread' becomes 'Brot' ('bread' in German).
'for' means one lot of letters go next to another (I've seen this in other clues).
'french mathematician' becomes 'mande' (I can't explain this - if you can you should believe this answer much more).
'l'+'brot'='lbrot'
'lbrot' put after 'mande' is 'MANDELBROT'.
Can you help me to learn more?
(Another definition for mandelbrot that I've seen is " Mathematician associated with fractal plotted set".)