Strokes substantial ego (6)
I believe the answer is:
solidi
'strokes' is the definition.
(plural of solidus, a slash or stroke)
'substantial ego' is the wordplay.
'substantial' becomes 'solid' (synonyms).
'ego' becomes 'I' ('I' in Latin).
'solid'+'i'='SOLIDI'
(Other definitions for solidi that I've seen before include "Oblique strokes" , "Slashes" , "old gold coins" .)