Underground omen about timepiece (9)
I believe the answer is:
metronome
'timepiece' is the definition.
(thesaurus)
'underground omen about' is the wordplay.
'underground' becomes 'metro' (synonyms).
'about' indicates an anagram.
'omen' with letters rearranged gives 'nome'.
'metro'+'nome'='METRONOME'
(Other definitions for metronome that I've seen before include "Musician's timekeeper" , "Instrument for beating time in music" , "Device that sounds tempo for music practice" , "A pacemaker" , "Musical timekeeper" .)