A sort of solid money (5)
I believe the answer is:
soldi
'money' is the definition.
(I have seen 'old money' mean 'soldi' so perhaps 'money' could also mean 'soldi')
'a sort of solid' is the wordplay.
'a sort of' is an anagram indicator.
'solid' with letters rearranged gives 'SOLDI'.
(Other definitions for soldi that I've seen before include "old money" , "Old Italian coins" .)