Sixteen working on annex (9)
I believe the answer is:
extension
'annex' is the definition.
(I know that annex can be written as extension)
'sixteen working on' is the wordplay.
'working' indicates an anagram.
'sixteen' is an anagram of 'extensi'.
'extensi'+'on'='EXTENSION'
(Other definitions for extension that I've seen before include "telephone number" , "Additional time for repayment" , "more rooms in house maybe" , "Office phone, perhaps" , "new wing" .)