A solemn proposal brought about love's consummation (10)
I believe the answer is:
apotheosis
'consummation' is the definition.
(both can mean when something is perfected)
'a solemn proposal brought about love's' is the wordplay.
'solemn' becomes 'po' (as in po-faced).
'proposal' becomes 'thesis' (I've seen this in another clue).
'brought about' is an insertion indicator.
'love' becomes 'o' (love means zero in tennis).
'thesis' going around 'o' is 'theosis'.
'a'+'po'+'theosis'='APOTHEOSIS'
(Other definitions for apotheosis that I've seen before include "sublimation" , "Perfect example" , "Highest point of development" , "crowning moment" , "Elevation to divine status" .)