Is he saintly? Yes, if only on Monday! (5)
I believe the answer is:
simon
'is he' is the definition.
'saintly? yes if only on monday' is the wordplay.
I cannot quite understand how this works, but
'on' is present in the answer.
'monday' could be 'm' and 'm' is found in the answer.
The remaining letters 'si' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for simon that I've seen before include "- and Garfunkel" , "Simple chap" , "See 1" , "Simple pieman" , "Apostle Peter's other name" .)