Crossword Genius

Completely low voice good with number (10)

Ross

I believe the answer is:

altogether

'completely' is the definition.
(synonyms)

'low voice good with number' is the wordplay.
'low voice' becomes 'alto' (I've seen this before).
'good' becomes 'g' (abbreviation).
'with' says to put letters next to each other.
'number' becomes 'ether' (I've seen this before).
'alto'+'g'+'ether'='ALTOGETHER'

(Other definitions for altogether that I've seen before include "nude in this" , "totally" , "Entirely" , "Wholly" , "Quite" .)

I've seen this clue in The Times.
Want a hint initially instead of a full solution? Install my app