A little is found in iridium and special substances (7)

'substances' is the definition.
Although both the answer and definition are plural nouns, I cannot see how they can define each other.

'a little is found in iridium and special' is the wordplay.
'a little' becomes 'some' (both can mean a small amount).
'is found in' means one lot of letters goes inside another.
'iridium' becomes 'Ir' (Ir is the chemical symbol for iridium).
'and' means one lot of letters go next to another.
'special' becomes 'S' (common abbreviation as in Special Air Service).
'some' inserted into 'ir' is 'isomer'.
'isomer'+'s'='ISOMERS'

Can you help me to learn more?