Blackadder's last to take advantage of cunning plan (4)
I believe the answer is:
ruse
'cunning plan' is the definition.
(I've seen this before)
'blackadder's last to take advantage' is the wordplay.
'last' indicates one should take the final letters.
'to take advantage' becomes 'use' (I've seen this in another clue).
The last letter of 'blackadder' is 'r'.
'r'+'use'='RUSE'
'of' acts as a link.
(Other definitions for ruse that I've seen before include "Sure to be a plot" , "Clever stratagem or plan" , "Trick, stratagem" , "Deceptive device" , "Deceptive manoeuvre" .)