Like fast food? Perhaps given porridge to eat at first riles one (9)

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

( STIR [prison – porridge is a slang term for serving a prison sentence] + FED [given something to eat] – so STIR FED can be defined as ‘given porridge’) containing (to eat) ( R [ initial letter of {at first} R ILES] + I [Roman numeral for one])