### Clue: How much of the main substance is present? It's madness to include lithium (8)
### Answer: SALINITY 
 ### Explanation: Definition: How much of the main substance is present
"the main"=the sea, so "the main substance" is salt. Not quite sure how to parse the wordplay – LI[thium], included **in** SANITY, with **in**SANITY="madness"?