The most common and most likely answer to this crossword clue is the 11 letter word COUNTERMINE
C
O
U
N
T
E
R
M
I
N
E
Buddy explains!

The straight part
We know that "Frustrate plot" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case frustrate plot = countermine.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does COUNTERMINE mean?
COUNTERMINE noun- A mine used by defenders to intercept an enemy mine or tunnel.
- An underground gallery excavated to intercept and destroy the mining of an enemy.
- A stratagem or plot by which another stratagem or project is defeated.
- To plot opposition; to frustrate the initiatives of another.
- To sap hostile mining.