Fool on sleeper is a criminal (9)
I believe the answer is:
kidnapper
'criminal' is the definition.
(I know that kidnapper is a type of criminal)
'fool on sleeper' is the wordplay.
'fool' becomes 'kid' (kidding is a kind of fooling).
'on sleeper' becomes 'napper' (I am not sure about this - if you are sure you should believe this answer much more).
'kid'+'napper'='KIDNAPPER'
'is a' acts as a link.
Can you help me to learn more?
(Other definitions for kidnapper that I've seen before include "One wrongly takes" , "criminal" , "Captor of a human being, for ransom" , "one to get child removed?" , "serious offender" .)