Look back at horse of French king (6)
I believe the answer is:
gander
'look' is the definition.
(informal term for a look or glance)
'back at horse of french king' is the wordplay.
'back at horse' becomes 'gan' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'of french' becomes 'de' ('of' in French).
'king' becomes 'R' (abbreviation for rex, king in Latin).
'gan'+'de'+'r'='GANDER'
Can you help me to learn more?
(Other definitions for gander that I've seen before include "He has webbed feet" , "glance" , "Butchers - goose" , "Male bird - look" , "Male goose; look (colloq.)" .)