Family joins German ruler (4)
I believe the answer is:
king
'ruler' is the definition.
(I know that king is a type of crowned head)
'family joins german' is the wordplay.
'family' becomes 'kin' (synonyms).
'joins' means one lot of letters go next to another.
'german' becomes 'g' (abbreviation).
'kin'+'g'='KING'
(Other definitions for king that I've seen before include "Most important chessman" , "Sovereign" , "Man with a crown" , "and 21 Across: Shakespeare play" , "initially unable to move" .)