Check a king; a German one (4)
I believe the answer is:
rein
'check' is the definition.
(both can mean to control or limit)
'a king a german one' is the wordplay.
'a king' becomes 'R' (abbreviation for rex, king in Latin).
'a german one' becomes 'ein' (I've seen this in another clue).
'r'+'ein'='REIN'
(Other definitions for rein that I've seen before include "Check; control" , "Restriction" , "Restraining lead" , "Curb" , "A means of control" .)