Steer round extremely nasty stone (4)
I believe the answer is:
onyx
'stone' is the definition.
(I've seen this before)
'steer round extremely nasty' is the wordplay.
'steer' becomes 'ox'.
'round' is an insertion indicator.
'extremely' means to remove the middle letters.
'nasty' with its centre removed is 'ny'.
'ox' enclosing 'ny' is 'ONYX'.
(Other definitions for onyx that I've seen before include "Smooth stone with bands of colour" , "Mineral with coloured layers" , "Banded chalcedony" , "Agate with different bands of colour" , "Stone-like marble with layered colours" .)