Earth around Pompeii's first ruin (5)
I believe the answer is:
spoil
'ruin' is the definition.
(I know that ruin is a more specific form of spoil)
'earth around pompeii's first' is the wordplay.
'earth' becomes 'soil' (earth can mean soil or dirt).
'around' is an insertion indicator.
'pompeii's first' becomes 'p' (sst letter of 'pompeii').
'soil' enclosing 'p' is 'SPOIL'.
(Other definitions for spoil that I've seen before include "Cook" , "Destroy or ruin" , "Treat overindulgently" , "Impair" , "Deface" .)