With last bit of peroxide, denture could be sterilised (8)

'sterilised' is the definition.
(neutering is a kind of sterilizing)

'with last bit of peroxide denture could be' is the wordplay.
'with' means one lot of letters goes inside another.
'last bit of' says to take the final letters.
'could be' indicates an anagram.
The last letter of 'peroxide' is 'e'.
'denture' anagrammed gives 'neuterd'.
'e' put inside 'neuterd' is 'NEUTERED'.