A lord, a saint and a well-known swimmer (5)
I believe the answer is:
steel
'a lord' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'a saint and a well-known swimmer' is the wordplay.
'a saint' becomes 'st' (abbreviation).
'and' is a charade indicator (letters next to each other).
'a well-known swimmer' becomes 'eel' (eels swim. I am not sure about the 'well-known' bit.).
'st'+'eel'='STEEL'
Can you help me to learn more?
(Other definitions for steel that I've seen before include "Knife-sharpener" , "May be cold" , "Iron/carbon alloy" , "Hard, strong, grey alloy" , "Strength, determination" .)