Material for filling in the morning magazine the French rejected (7)
I believe the answer is:
amalgam
'material for filling' is the definition.
'amalgam' can be an answer for 'filling' (amalgam is an alloy used as a dental filling). I'm not certain of the 'material for' bit.
'morning magazine the french rejected' is the wordplay.
'morning' becomes 'am' (as in 12:00 AM).
'magazine' becomes 'mag' (abbreviation for magazine).
'the french' becomes 'la' (one of the French words for 'the').
'rejected' is a reversal indicator.
'mag'+'la'='magla'
'magla' written backwards gives 'algam'.
'am'+'algam'='AMALGAM'
'in the' is the link.
Can you help me to learn more?
(Other definitions for amalgam that I've seen before include "Blend of metals" , "Mixture (for dental fillings)" , "often filling" , "Ring-bearing hobbit" , "Mercury alloy - combination" .)