My French king is brother (4)
I believe the answer is:
monk
'is brother' is the definition.
(I've seen this before)
'my french king' is the wordplay.
'my french' becomes 'mon'.
'king' becomes 'k'.
'mon'+'k'='MONK'
(Other definitions for monk that I've seen before include "Tonsured person" , "Friar" , "Setter" , "One vowed" , "Man in a religious order" .)