Happens to upset company dogs (6)
I believe the answer is:
occurs
'happens' is the definition.
('occur' can be a synonym of 'happen')
'upset company dogs' is the wordplay.
'upset' says the letters should be written in reverse.
'company' becomes 'co' (abbreviation).
'dogs' becomes 'curs' (cur is a kind of dog).
'co' back-to-front is 'oc'.
'oc'+'curs'='OCCURS'
'to' acts as a link.
(Other definitions for occurs that I've seen before include "Happens, takes place" , "Comes to pass" , "Happens or comes to mind" , "Crocus (anag)" .)