Slaughter ran out of control in prison (7)
I believe the answer is:
carnage
'slaughter' is the definition.
(I know that slaughter can be written as carnage)
'ran out of control in prison' is the wordplay.
'out of control' is an anagram indicator.
'in' means one lot of letters goes inside another.
'prison' becomes 'cage' (cage is a kind of prison).
'ran' is an anagram of 'rna'.
'rna' placed into 'cage' is 'CARNAGE'.
(Other definitions for carnage that I've seen before include "'Massacre, slaughter (7)'" , "Mass slaughter" , "Wholesale killing" , "The killing of many people, slaughter" , "Bloody slaughter as in battle" .)