Heading west in small unexplored ravine (5)
I believe the answer is:
nulla
'ravine' is the definition.
(I've seen this before)
'heading west in small unexplored' is the wordplay.
'heading west' says the letters should be written backwards.
'in' says the answer is hidden in the clue.
'allun' can be found hidden inside 'small unexplored'.
'allun' in reverse letter order is 'NULLA'.
(Other definitions for nulla that I've seen before include "ravine" , "Stream" .)