Slashes independent firm first (6)
I believe the answer is:
solidi
'slashes' is the definition.
('solidus' can be a synonym of 'slash')
'independent firm first' is the wordplay.
'independent' becomes 'i'.
'firm' becomes 'solid' ('solid' can be a synonym of 'firm'**).
'first' means one lot of letters go next to another.
'i' put after 'solid' is 'SOLIDI'.
(Other definitions for solidi that I've seen before include "old gold coins" , "Oblique strokes" , "Slashes" .)