Add-on for individual office telephone (9)
I believe the answer is:
extension
This could be a double definition.
'add-on' is the first definition.
(I know that addon can be written as extension)
'individual office telephone' is the second definition.
'extension' can be an answer for 'telephone' (extension is a kind of telephone). I am unsure of the 'individual office' bit.
'for' is the link.
Can you help me to learn more?
(Other definitions for extension that I've seen before include "Added part" , "telephone number" , "new part of home?" , "more rooms in house maybe" , "Additional rooms" .)