'Number Ten' ace has it worked out (11)
I believe the answer is:
anaesthetic
'number' is the definition.
(I've seen this before)
'ten ace has it worked out' is the subsidiary indication.
'worked out' is an anagram indicator.
'ten'+'ace'+'has'+'it'='tenacehasit'
'tenacehasit' anagrammed gives 'anaesthetic'.
(Other definitions for anaesthetic that I've seen before include "used in theatre" , "Theatre number" , "Drug that causes temporary loss of feeling" , "painkiller" , "A number of theatres" .)