Scowl when I am in prayer (7)
I believe the answer is:
grimace
'scowl' is the definition.
(thesaurus)
'i am in prayer' is the wordplay.
'i am' becomes 'im'.
'in' is an insertion indicator.
'prayer' becomes 'grace' (grace is a kind of prayer).
'im' put inside 'grace' is 'GRIMACE'.
'when' is the link.
(Other definitions for grimace that I've seen before include "Frown" , "Expression of pain or disgust" , "Dour" , "displeasure" , "Distortion of the face" .)