Group of countries taking part in a tournament (4)
I believe the answer is:
nato
'group of countries' is the definition.
(North Atlantic Treaty Organisation)
'part in a tournament' is the wordplay.
'part' says the answer is hidden in the clue.
'NATO' is hidden in the letters of 'in a tournament'.
'taking' acts as a link.
(Other definitions for nato that I've seen before include "Atlantic alliance (abbr.)" , "Western mil. alliance" , "Military alliance (acronym)" , "Security agreement" , "Military bloc" .)