Very old partner causing concern (3)
I believe the answer is:
vex
'concern' is the definition.
(thesaurus)
'very old partner' is the wordplay.
'very' becomes 'v' (abbreviation).
'old partner' becomes 'ex' (an ex-partner).
'v'+'ex'='VEX'
'causing' acts as a link.
(Other definitions for vex that I've seen before include "Irritate, annoy" , "Irk" , "upset" .)