Crack dealer finally arrested in Strand (6)
I believe the answer is:
breach
'crack dealer' is the definition.
'breach' can be an answer for 'crack' (thesaurus). I am unsure of the 'dealer' bit.
'finally arrested in strand' is the wordplay.
'finally arrested' becomes 'r' (I can't justify this - if you can you should believe this answer much more).
'in' is an insertion indicator.
'strand' becomes 'beach' (I've seen this before).
'r' going inside 'beach' is 'BREACH'.
Can you help me to learn more?
(Other definitions for breach that I've seen before include "Violation, maybe of the peace" , "'Gap, break or hole (6)'" , "Contravention" , "Break (in a wall, etc)" , "part of firearm, say" .)