Clue: A Catholic grandee, severest for the most part in liturgy, say (3,2,7)
Answer: ACT OF WORSHIP
Explanation: Definition: liturgy, say
A charade of 'a' plus C ('Catholic') plus TOF[f] WORS[t] ('grandee severest') minus the last letter of both words ('for the most part' – rather loose, if I am reading the clue correctly) plus HIP ('in').