Simple line crossed by criminal (4)
I believe the answer is:
bald
'simple' is the definition.
(thesaurus)
'line crossed by criminal' is the wordplay.
'line' becomes 'l' (used when specifying particular lines from a poem).
'crossed by' indicates putting letters inside (inserted letters are crossed or covered by others).
'criminal' becomes 'bad' (I've seen this in another clue).
'l' going within 'bad' is 'BALD'.
(Other definitions for bald that I've seen before include "With no effort to conceal" , "Without hair" , "Unadorned" , "With the tread worn away" , "Not covered with hair" .)