Magazine to investigate many European leaders (4)
I believe the answer is:
time
'magazine' is the definition.
(US magazine)
'to investigate many european leaders' is the wordplay.
'leaders' suggests taking the first letters.
The initial letters of 'to investigate many european' is 'TIME'.
(Other definitions for time that I've seen before include "Indefinite period" , "The father of Truth, presumably" , "Prison sentence; tempo" , "Tempo; occasion" , "Bird" .)