Clue: Service breaks in general essentially all together (2,5)
Answer: EN MASSE
Explanation: Definition: all together
An envelope ('breaks in') of MASS ('service') in ENE ("g**ENE**ral essentially' – a little off-centre). *As DougV @1 & 2 points out, moving the 'in' to 'in g**ENE**ral' places the particle squarely in the centre.