Warning bolshie, loud prisoner (3,4)
I believe the answer is:
red flag
'warning' is the definition.
(I've seen this before)
'bolshie loud prisoner' is the wordplay.
'bolshie' becomes 'red'.
'loud' becomes 'f' (short for forte - 'loud' in music).
'prisoner' becomes 'lag' (I've seen this before).
'red'+'f'+'lag'='RED FLAG'
(Other definitions for red flag that I've seen before include "The ... ..., anthem of the British Labour Party" , "That should make you careful!" , "A number of revolutionaries" , "Emblem of socialist revolution or danger" , "Warning to stop" .)