The late converted into the quick? (7)
I believe the answer is:
athlete
'the quick?' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'the late converted' is the wordplay.
'converted' indicates an anagram.
'the'+'late'='thelate'
'thelate' is an anagram of 'ATHLETE'.
'into' is the link.
Can you help me to learn more?
(Other definitions for athlete that I've seen before include "Eg long jumper" , "Sportsman in physical contests" , "One who engages in physical sports" , "Person fit for physical sports" , "Person trained to compete in sports" .)