Cooler - perhaps it's going to rain (4)
I believe the answer is:
'cooler' is the definition.
(both are slang terms for prison)
'perhaps it's going to rain' is the wordplay.
'perhaps' is an anagram indicator.
'going to' is a charade indicator (letters next to each other).
'rain' becomes 'r' (this could be a standard abbreviation which I don't know about).
'its' anagrammed gives 'sti'.
Can you help me to learn more?
(Other definitions for stir that I've seen before include "Cause trouble" , "song and dance" , "unrest" , "Agitate; wake from sleep" , "Uproar" .)