Sensible grade by teacher of French is put up (5-6)
I believe the answer is:
Here is my best explanation:
'sensible' is the definition.
(I've seen this before)
'grade by teacher of french is put up' is the subsidiary indication.
'grade' becomes 'level' ('level' can be a synonym of 'grade'**).
'by' is a charade indicator (letters next to each other).
'teacher' becomes 'head'.
'of french' becomes 'de'.
'is put up' says the letters should be written backwards.
'de' backwards is 'ed'.
(Other definitions for level-headed that I've seen before include "Sensible", "Calm and sensible" and "Showing good judgement".)