Simple student taken in by criminal (4)
I believe the answer is:
bald
'simple' is the definition.
(thesaurus)
'student taken in by criminal' is the wordplay.
'student' becomes 'l' (as in L-plates for learner drivers).
'taken in by' means one lot of letters goes inside another.
'criminal' becomes 'bad' (**).
'l' inserted within 'bad' is 'BALD'.
(Other definitions for bald that I've seen before include "Plain; hairless" , "Follicly challenged?" , "Exposed" , "With no effort to conceal" , "Not covered with hair" .)