The saintly one will astound you! (4)
I believe the answer is:
stun
'astound you' is the definition.
'stun' can be an answer for 'astound' (I've seen this before). I am not sure about the 'you' bit.
'the saintly one will' is the wordplay.
I cannot quite understand how this works, but
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is found in the answer.
'one' could be 'un' (northern English dialect form of 'one') and 'un' is found within the answer.
A single letter 's' remains which might be clued in a way I don't understand.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for stun that I've seen before include "Stagger, amaze" , "Daze, concuss" , "Confuse" , "Surprise greatly, knock someone''s socks off" , "Daze, knock senseless" .)