Part — of backward Time Lord? (4)
'part' is the definition.
'of backward time lord?' is the wordplay.
'backward' says the letters should be written backwards.
'of' indicates a hidden word.
'elor' is hidden within 'time lord'.
'elor' in reverse letter order is 'ROLE'.
(Other definitions for role that I've seen before include "Part in play, film etc", "Impersonation", "Playing part", "A part of lore", "Duty".)