Perfect old instrument with a brass casing on top (8)
I believe the answer is:
absolute
'perfect' is the definition.
(synonyms)
'old instrument with a brass casing on top' is the wordplay.
'old instrument' becomes 'lute' (medieval stringed instrument).
'with' means one lot of letters go next to another.
'casing' says to hollow out the word (remove centre letters) (the outside letters are the case of the word).
'top' suggests taking the first letters (in a down clue, the first letter is top).
'brass' with its centre removed is 'bs'.
The initial letter of 'on' is 'o'.
'a'+'bs'+'o'='abso'
'lute' put after 'abso' is 'ABSOLUTE'.
(Other definitions for absolute that I've seen before include "Despotic" , "Total; not relative" , "Not qualified or diminished in any way" , "-273.15degC" , "Complete or not limited" .)