Worship with water coming in? That's completely stupid (9)
I believe the answer is:
brainless
'completely stupid' is the definition.
'worship with water coming in?' is the wordplay.
'worship with' becomes 'bless' (I am not sure about this - if you are sure you should believe this answer much more).
'water' becomes 'rain' (rain is a kind of water).
'coming in?' indicates putting letters inside.
'bless' enclosing 'rain' is 'BRAINLESS'.
'that's' acts as a link.
(Other definitions for brainless that I've seen before include "dim" , "Devoid of intelligence" , "Stupid, foolish" , "Silly" .)