Running business is becoming worry (5,7)
I believe the answer is:
going concern
'running business' is the definition.
(a 'going concern' is a profitable business)
'becoming worry' is the wordplay.
'becoming' becomes 'going' ('go' can be a synonym of 'become').
'worry' becomes 'concern' ('concern' can be a synonym of 'worry').
'going'+'concern'='GOING-CONCERN'
'is' is the link.
(Other definitions for going concern that I've seen before include "A continuing business" , "successful business" , "profitable business" , "Viable business" , "Thriving business" .)