Friends collecting a sum for the church (5)

'friends' is the definition.
(chum is a kind of friend)

'collecting a sum for the church' is the wordplay.
I cannot really understand how this works, but
an anagram of 'sum' is 'ums' which is located in the answer.
'church' could be 'ch' (common abbreviation) and 'ch' is found within the answer.
No letters remain.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?