Plain but audible footfall (6)
I believe the answer is:
steppe
'plain' is the definition.
(I know that steppe is a type of plain)
'audible footfall' is the wordplay.
'audible' shows a homophone (sound like).
'footfall' becomes 'step' (I've seen this in another clue).
'step' sounds like 'STEPPE'.
'but' acts as a link.
(Other definitions for steppe that I've seen before include "Dry plain of Eurasia" , "Large treeless area of Siberia" , "Dry grassy plain of central Europe" , "(Russian) grassy plain" , "crosses Siberia" .)