Split appearing among upwardly mobile Microsoft Fellows (4)
I believe the answer is:
left
'split' is the definition.
(I've seen this before)
'among upwardly mobile microsoft fellows' is the wordplay.
'upwardly mobile' says the letters should be written backwards.
'among' indicates the answer is hidden within the clue.
'tfel' is hidden in the letters of 'microsoft fellows'.
'tfel' backwards is 'LEFT'.
'appearing' acts as a link.
(Other definitions for left that I've seen before include "Socialist direction" , "split" , "The side of socialism" , "Gone away - remaining" , "political grouping" .)