A number of characters committed a crime setting up trainee officer (5)
I believe the answer is:
cadet
'trainee officer' is the definition.
(I've seen this before)
'a number of characters committed a crime' is the wordplay.
'number' becomes 'c' (C is an example).
'of characters' indicates anagramming the letters (I've seen 'characters' mean this).
'committed a crime' becomes 'det' (I can't justify this - if you can you should believe this answer much more).
'a'+'c'='ac'
'ac' is an anagram of 'ca'.
'ca'+'det'='CADET'
'setting up' is the link.
This may not be right. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for cadet that I've seen before include "not the heir" , "Police or armed forces trainee" , "Military student" , "One in training" , "junior role" .)