Blackadder finally trapped by silly plan (5)
I believe the answer is:
draft
'plan' is the definition.
(drafting is a kind of planning)
'blackadder finally trapped by silly' is the wordplay.
'finally' says to take the final letters.
'trapped by' indicates putting letters inside.
'silly' becomes 'daft' (I've seen this before).
The final letter of 'blackadder' is 'r'.
'r' inserted into 'daft' is 'DRAFT'.
(Other definitions for draft that I've seen before include "Outline or sketch" , "Formulate" , "Early version of text" , "plan outlined" , "Preliminary drawing" .)