Launch religious sect, taking a knock (8)
I believe the answer is:
catapult
'launch' is the definition.
(I've seen this before)
'religious sect taking a knock' is the wordplay.
'religious sect' becomes 'cult' (I've seen this in another clue).
'taking' means one lot of letters goes inside another.
'knock' becomes 'tap' (synonyms).
'a'+'tap'='atap'
'cult' enclosing 'atap' is 'CATAPULT'.
(Other definitions for catapult that I've seen before include "Stone thrower" , "Child's stone shooter" , "Hurl forcibly as from sling" , "Stone-hurling weapon" , "It's a weapon with range" .)