A solemn proposal brought about love's consummation (10)
I believe the answer is:
'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'.
(Other definitions for apotheosis that I've seen before include "Given maximum promotion", "ideal", "Deification", "Perfect example", "glorification".)