Winners of a complicated case (4)
I believe the answer is:
aces
'winners' is the definition.
(I've seen this in another clue)
'a complicated case' is the wordplay.
'a complicated' indicates an anagram (a complex or confusing ordering of the letters).
'case' is an anagram of 'ACES'.
'of' is the link.
(Other definitions for aces that I've seen before include "Playing cards with single pips on their faces" , "Winning serves" , "holes in one" , "passes with flying colours in US" , "Top cards or people" .)