### Clue: Galilean satellite first penetrating rings (8)
### Answer: CALLISTO 
 ### Explanation: Definition: Galilean satellite
 IST(1st/abbrev. for "first", with the Roman numeral substitution) **contained in**(penetrating) [CALL,O](2 rings, the first on the telephone, and the second the letter representing a circle/a ring).
Defn: A satellite/moon of Jupiter, discovered by Galileo Galilei.