There's a snag when you knock the church (5)
I believe the answer is:
hitch
'there's a snag' is the definition.
(synonyms)
'knock the church' is the wordplay.
'knock' becomes 'hit' (synonyms).
'the church' becomes 'ch' (common abbreviation).
'hit'+'ch'='HITCH'
'when you' acts as a link.
This may not be correct. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for hitch that I've seen before include "Hindrance" , "Snag - free ride" , "Unforeseen obstacle" , "something uplifting" , "Small problem" .)