Officer in prison riot reveals venomous creature (8)
I believe the answer is:
scorpion
'venomous creature' is the definition.
(I've seen this before)
'officer in prison riot' is the wordplay.
'officer' becomes 'co' (Commanding Officer abbreviation).
'in' means one lot of letters goes inside another.
'riot' is an anagram indicator.
'prison' anagrammed gives 'srpion'.
'co' inserted within 'srpion' is 'SCORPION'.
'reveals' is the link.
(Other definitions for scorpion that I've seen before include "Arachnid with venomous sting" , "Stinger with a long tail" , "Small spider with pincers and stinging tail" , "Kind of arachnid with pincers and stinging tail" , "Kind of spider with pincers and stinging tail" .)