Underground banker, no good as timekeeper (9)
I believe the answer is:
metronome
'timekeeper' is the definition.
(I have seen 'Musical timekeeper ' mean 'metronome' so perhaps 'timekeeper' could also mean 'metronome')
'underground banker no good' is the wordplay.
'underground' becomes 'metro' (synonyms).
'banker' becomes 'gnome' (gnome can informally mean a powerful banker).
'no' is a deletion indicator.
'good' becomes 'g' (abbreviation).
'gnome' with 'g' taken out is 'nome'.
'metro'+'nome'='METRONOME'
'as' is the link.
(Other definitions for metronome that I've seen before include "Musical timekeeper" , "Musician's time device" , "Device to indicate tempo" , "Ticker setting tempo of music" , "A pacemaker" .)