Diana's meal-time prayer? Shame! (8)
I believe the answer is:
disgrace
'shame' is the definition.
(I know that shame can be written as disgrace)
'diana's meal-time prayer?' is the wordplay.
'diana's' becomes 'Di's' (Di is short for Diana).
'meal-time prayer?' becomes 'grace' (I've seen this before).
'dis'+'grace'='DISGRACE'
(Other definitions for disgrace that I've seen before include "Bring shame upon" , "Shame or loss of respect" , "Cause of shame or dishonour" , "Dishonour, ignominy" , "Loss of reputation as the result of dishonourable action" .)