Threatening words when loser upset over conclusion of referee (2,4)
I believe the answer is:
or else
'threatening words' is the definition.
(do what I say... or else!)
'loser upset over conclusion of referee' is the wordplay.
'upset' indicates anagramming the letters.
'over' indicates putting letters inside.
'conclusion of' indicates one should take the final letters.
The final letter of 'referee' is 'e'.
'loser' is an anagram of 'orlse'.
'orlse' placed around 'e' is 'OR ELSE'.
'when' is the link.
(Other definitions for or else that I've seen before include "Vague threat" , "Alternatively" , "Otherwise - that's a threat" , "Instead of" , "Threatening phrase" .)