Saint, initially, in fall from grace (3)
I believe the answer is:
sin
'fall from grace' is the definition.
(I've seen this before)
'saint initially in' is the wordplay.
'initially' indicates taking the first letters.
The initial letter of 'saint' is 's'.
's'+'in'='SIN'
(Other definitions for sin that I've seen before include "Behave badly" , "Evil" , "Breach of morality" , "'Transgression, mortal or venial (3)'" , "Wicked deed" .)