Strained relations with former partner over telephone (9)
I believe the answer is:
extension
'telephone' is the definition.
(extension is a kind of telephone)
'strained relations with former partner' is the wordplay.
'strained relations' becomes 'tension' (I can't justify this - if you can you should believe this answer much more).
'with' says to put letters next to each other.
'former partner' becomes 'ex' (an ex-partner).
'tension' put after 'ex' is 'EXTENSION'.
'over' is the link.
Can you help me to learn more?
(Other definitions for extension that I've seen before include "telephone number" , "Additional rooms" , "time added on" , "more rooms in house maybe" , "supplement" .)