Totally high voice from grand old anaesthetic (10)
I believe the answer is:
altogether
'totally' is the definition.
(synonyms)
'high voice from grand old anaesthetic' is the wordplay.
'high voice' becomes 'alto' (I've seen this before).
'from' means one lot of letters go next to another.
'grand' becomes 'g' (G is short for grand as in a thousand).
'old anaesthetic' becomes 'ether' (I've seen this before).
'alto'+'g'+'ether'='ALTOGETHER'
(Other definitions for altogether that I've seen before include "totally" , "Wholly" , "special to get Hermes trousers" , "Completely, on the whole" , "Entirely" .)