### Clue: Union probing phone system's boss (4)
### Answer: STUD 
 ### Explanation: Definition: boss
 U(abbrev. for "Union") **contained in**(probing) STD(abbrev. for "Subscriber Trunk Dialling", a phone system allowing subscribers to make trunk calls without operator assistance).
Defn: …/a piece of metal projecting from the centre of a shield.