Underground no-good creature of myth that will move in time (9)
I believe the answer is:
'that will move in time' is the definition.
(metronomes keep time for musicians)
'underground no-good creature of myth' is the wordplay.
'underground' becomes 'metro' (synonyms).
'no' indicates named letters should be taken away.
'good' becomes 'g' (abbreviation).
'creature of myth' becomes 'gnome' (gnomes are folklore creatures).
'gnome' with 'g' taken out is 'nome'.
(Other definitions for metronome that I've seen before include "Device to indicate tempo" , "Instrument for beating time in music" , "A pacemaker" , "Device to help regulate tempo" , "Musician's time device" .)