Though saintly, is wrong one day (5)
I believe the answer is:
simon
'though saintly' is the definition.
I can't tell whether this defines the answer.
'is wrong one day' is the wordplay.
'wrong' indicates an anagram.
'one day' becomes 'mon' (abbreviation for Monday).
'is' anagrammed gives 'si'.
'si'+'mon'='SIMON'
Can you help me to learn more?
(Other definitions for simon that I've seen before include "- and Garfunkel" , "- Templar, The Saint" , "Fellow" , "Simple chap" , "A simple character in nursery rhyme" .)