Good number led by singer completely (10)
I believe the answer is:
altogether
'completely' is the definition.
(synonyms)
'good number led by singer' is the wordplay.
'good' becomes 'g' (abbreviation).
'number' becomes 'ether' (anaesthetic, something which numbs).
'led by' means one lot of letters go next to another (some letters go in front of others).
'singer' becomes 'alto' (type of singing voice).
'g'+'ether'='gether'
'gether' after 'alto' is 'ALTOGETHER'.
(Other definitions for altogether that I've seen before include "Quite" , "totally" , "Entirely" , "Wholly" , "Completely, in total" .)