Earth around Portsmouth's top ruin (5)
I believe the answer is:
'ruin' is the definition.
(I know that ruin is a more specific form of the action spoil)
'earth around portsmouth's top' is the wordplay.
'earth' becomes 'soil' (earth can mean soil or dirt).
'around' means one lot of letters goes inside another.
'top' indicates taking the first letters (in a down clue, the first letter is top).
The initial letter of 'portsmouth' is 'p'.
'soil' going around 'p' is 'SPOIL'.
(Other definitions for spoil that I've seen before include "Treat with excessive kindness" , "Mess up" , "baby" , "Cook" , "Be too nice to - strip" .)