Criminal takes over royal line (6)
I believe the answer is:
streak
'line' is the definition.
(thesaurus)
'criminal takes over royal' is the wordplay.
'criminal' indicates an anagram.
'over' means one lot of letters goes inside another.
'royal' becomes 'r' (abbreviation).
'takes' is an anagram of 'steak'.
'steak' enclosing 'r' is 'STREAK'.
(Other definitions for streak that I've seen before include "Long thin stripe - run naked" , "Go faster" , "Strip of a different colour" , "Run naughtily nude in public" , "Stripe of contrasting colour" .)