Singer initially given anaesthetic? Absolutely (10)
I believe the answer is:
altogether
'absolutely' is the definition.
('altogether' can be a synonym of 'absolutely')
'singer initially given anaesthetic?' is the wordplay.
'singer' becomes 'alto' (alto is a kind of singer).
'initially' says to take the initial letters.
'anaesthetic?' becomes 'ether' (ether is a kind of anaesthetic).
The first letter of 'given' is 'g'.
'alto'+'g'+'ether'='ALTOGETHER'
(Other definitions for altogether that I've seen before include "Completely, in total" , "Entirely" , "Quite" , "totally" , "Wholly" .)