Union probing phone system's boss (4)
I believe the answer is:
stud
'boss' is the definition.
(both can mean a metal decoration)
'union probing phone system's' is the wordplay.
'union' becomes 'U' (common abbreviation as in 'Trades Union Congress').
'probing' means one lot of letters goes inside another (probe can mean to make a hole in).
'phone system' becomes 'std' (short for subscriber trunk dialling).
'u' going within 'std' is 'STUD'.
(Other definitions for stud that I've seen before include "Small rounded earring" , "card game" , "Playboy" , "Animal breeding farm" , "Boss; item of jewellery" .)