Filling morning with a large magazine to go over (7)
I believe the answer is:
amalgam
'filling' is the definition.
(amalgam is an alloy used as a dental filling)
'morning with a large magazine to go over' is the wordplay.
'morning' becomes 'am'.
'with' means one lot of letters go next to another.
'large' becomes 'L' (eg in clothes sizes).
'magazine' becomes 'mag' (abbreviation for magazine).
'to go over' shows that the letters should be reversed in order.
'mag' backwards is 'gam'.
'am'+'a'+'l'+'gam'='AMALGAM'
(Other definitions for amalgam that I've seen before include "Mixture, blend" , "Blend of metals" , "Mixture - alloy of mercury" , "Combination of diverse elements" , "Blend of diverse things" .)