Essential service needed for chain reaction (8,4)

I believe the answer is:

critical mass

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'chain reaction' is the definition.
Both the definition and answer are singular nouns.
Maybe there's a link between them I don't understand?

'essential service needed' is the wordplay.
'essential' becomes 'critical' (I've seen this before).
'service' becomes 'mass' (church service).
'needed' is a charade indicator (letters next to each other).
'critical'+'mass'='CRITICAL MASS'

'for' is the link.

Can you help me to learn more?

I've seen this clue in The Daily Express.

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