Gather religious establishment mostly suppresses ecstasy (7)
I believe the answer is:
convene
'gather' is the definition.
(convening is a kind of gathering)
'religious establishment mostly suppresses ecstasy' is the wordplay.
'religious establishment' becomes 'convent'.
'mostly' means to remove the last letter.
'suppresses' says to put letters next to each other (in a down clue, some letters go above others).
'ecstasy' becomes 'e' (informal term for the drug).
'convent' with its final letter removed is 'conven'.
'conven'+'e'='CONVENE'
(Other definitions for convene that I've seen before include "Mass" , "Call together" , "rally" , "Muster" , "Assemble (for meeting)" .)