One running after calories consumed in fat camp (7)

I haven't cracked this one yet — but the brilliant team at Fifteensquared have:

A charade of FACT, an envelope (‘in’) of C (‘calories consumed’ – ‘consumed’ because the capital C used of food is a kilocalorie – 1000 calories as the old unit in physics) in ‘fat’; plus I (‘one’) plus ON (‘running’), with ‘after’ indicating the order of the particles.