During prayer this person's to make a face (7)
I believe the answer is:
grimace
'make a face' is the definition.
(synonyms)
'during prayer this person's' is the wordplay.
'during' indicates putting letters inside.
'prayer' becomes 'grace' (grace is a kind of prayer).
'this person's' becomes 'I'm' (('I am' - 'this person' means the crossword setter).
'grace' enclosing 'im' is 'GRIMACE'.
'to' is the link.
(Other definitions for grimace that I've seen before include "Disgusted expression" , "Dour" , "Wry look" , "Distorted facial expression" , "Nasty expression" .)