A memorial for the most solemn individual (10)
I believe the answer is:
gravestone
'a memorial' is the definition.
(I know that gravestone is a type of memorial)
'most solemn individual' is the wordplay.
'most solemn' becomes 'gravest' ('grave' can be a synonym of 'solemn').
'individual' becomes 'one' (an individual is one person).
'gravest'+'one'='GRAVESTONE'
'for the' acts as a link.
(Other definitions for gravestone that I've seen before include "lapidary valediction" , "Churchyard memorial" , "Burial marker" , "superior to body" .)