Disparaging service providing what's needed to sustain fission (8,4)

I believe the answer is:

critical mass

'what's needed to sustain fission' is the definition.

'disparaging service' is the wordplay.
'disparaging' becomes 'critical' (I've seen this before).
'service' becomes 'mass' (I've seen this in other clues).
'critical'+'mass'='CRITICAL MASS'

'providing' is the link.

I've seen this clue in The Guardian.

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