Vital service needed to make chain reaction (8,4)

I believe the answer is:

critical mass

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'make chain reaction' is the definition.
The definition suggests a singular noun which matches the answer.

'vital service needed' is the wordplay.
'vital' becomes 'critical' (synonyms).
'service' becomes 'mass' (church service).
'needed' says to put letters next to each other.
'critical'+'mass'='CRITICAL MASS'

'to' is the link.

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I've seen this clue in The Daily Express.

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