Memorial service's opening in serious mood (10)
I believe the answer is:
gravestone
'memorial' is the definition.
(I know that gravestone is a type of memorial)
'service's opening in serious mood' is the wordplay.
'opening' indicates taking the first letters.
'in' indicates putting letters inside.
'serious' becomes 'grave' (both can mean solemn).
'mood' becomes 'tone' (tone is a kind of mood).
The first letter of 'services' is 's'.
'grave'+'tone'='gravetone'
's' put within 'gravetone' is 'GRAVESTONE'.
(Other definitions for gravestone that I've seen before include "superior to body" , "Burial marker" , "lapidary valediction" , "Churchyard memorial" .)