Revolutionary prisoner comprehends loud warning (3,4)
I believe the answer is:
red flag
'warning' is the definition.
(I've seen this before)
'revolutionary prisoner comprehends loud' is the wordplay.
I cannot really understand how this works, but
'revolutionary' could be 'red' (red can informally mean communist) and 'red' is located in the answer.
'prisoner' could be 'lag' (slang term) and 'lag' is found within the answer.
'loud' could be 'f' (short for forte - 'loud' in music) and 'f' is found within the answer.
No letters remain.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for red flag that I've seen before include "signal to stop" , "Instruction to halt" , "Danger signal - keep it flying high!" , "The ... ..., anthem of the British Labour Party" , "Warning signal" .)