### Clue: Painting on plaster that's dry and firm (5)
### Answer: SECCO 
 ### Explanation: Definition: Painting on plaster
SEC (dry) + CO (company, firm). Again rather weak, as "sec" and "secco" are essentially the same word. Secco is a technique of painting on dry plaster, in contrast to fresco, where it's still wet, so the clue is an extended definition