Saint revealing deception by old man (7)
I believe the answer is:
patrick
'saint' is the definition.
(I know that Patrick is a patron saint)
'deception by old man' is the wordplay.
'deception' becomes 'trick' (synonyms).
'by' says to put letters next to each other.
'old man' becomes 'pa' (both can mean someone's father).
'trick' after 'pa' is 'PATRICK'.
'revealing' is the link.
(Other definitions for patrick that I've seen before include "-- Moore, astronomer" , "Patron saint of Ireland" , "Ireland's patron saint" , "- - Swayze: film actor" , "Patron saint who said 'Shoo!' to serpents" .)