Morning gala upset over my first cavity filler (7)
I believe the answer is:
amalgam
'cavity filler' is the definition.
The definition and answer can be both substances as well as being singular nouns.
Perhaps you can see an association between them that I don't see?
'morning gala upset over my first' is the wordplay.
'morning' becomes 'am' (as in m2:00 AM).
'upset' indicates an anagram.
'over' is an insertion indicator.
'my first' becomes 'm' (1st letter of 'my').
'gala' anagrammed gives 'alga'.
'am' put inside 'alga' is 'amalga'.
'amalga'+'m'='AMALGAM'
Can you help me to learn more?
(Other definitions for amalgam that I've seen before include "mercurial combination" , "Combination of diverse elements" , "Mixture (for dental fillings)" , "Mixture - alloy of mercury" , "Mixture, blend" .)