Crackpot jailer cut loose for a prank (9,4)
I believe the answer is:
practical joke
'prank' is the definition.
(practical joke is a kind of prank)
'crackpot jailer cut loose' is the wordplay.
'cut' means to remove the last letter.
'loose' indicates an anagram (letters are loosened from their original positions).
'crackpot'+'jailer'='crackpotjailer'
'crackpotjailer' with its final letter taken off is 'crackpotjaile'.
'crackpotjaile' with letters rearranged gives 'PRACTICAL JOKE'.
'for a' acts as a link.
(Other definitions for practical joke that I've seen before include "rag" , "Unkind prank" , "Jape - hoax" , "Trick played to discomfort someone" , "Prank designed to make a fool of its victim" .)