Slashes substantial incomes, primarily (6)
I believe the answer is:
solidi
'slashes' is the definition.
('solidus' can be a synonym of 'slash')
'substantial incomes primarily' is the wordplay.
'substantial' becomes 'solid' ('solid' can be a synonym of 'substantial').
'primarily' says to take the initial letters.
The first letter of 'incomes' is 'i'.
'solid'+'i'='SOLIDI'
(Other definitions for solidi that I've seen before include "Slashes" , "Oblique strokes" , "old gold coins" .)