Criminal tries to leave Eastern prison (4)
I believe the answer is:
stir
'prison' is the definition.
(informal term for prison)
'criminal tries to leave eastern' is the wordplay.
'criminal' indicates an anagram.
'to leave' indicates named letters should be taken away.
'eastern' becomes 'e'.
'tries' with 'e' taken away is 'tris'.
'tris' anagrammed gives 'STIR'.
(Other definitions for stir that I've seen before include "created by an agitator?" , "Agitate; wake from sleep" , "Get moving" , "Mix the ingredients" , "Arouse (a memory)" .)