Serious offender (5)
I believe the answer is:
felon
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">serious offender</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for felon that I've seen before include "Serious criminal" , "convict" , "Has conviction" , "criminal from Texas" , "Person who has committed a serious crime" .)