Crack dealer finally seen in Strand (6)
I believe the answer is:
breach
'crack' is the definition.
(I know that crack has the is a more specific form of the action break)
'dealer finally seen in strand' is the wordplay.
'finally' suggests the final letters.
'seen in' indicates putting letters inside.
'strand' becomes 'beach' (I've seen this in another clue).
The last letter of 'dealer' is 'r'.
'r' going inside 'beach' is 'BREACH'.
(Other definitions for breach that I've seen before include "Disturbance as of the peace" , "Gap made in a fortification" , "Split, break" , "Break (in a wall, etc)" , "transgression" .)