The most common and most likely answer to this crossword clue is the 12 letter word EAVESDROPPED
I've seen this crossword clue in The Daily Telegraph
E
A
V
E
S
D
R
O
P
P
E
D
Buddy explains!
The straight part
We know that "monitored" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case monitored = eavesdropped.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 EAVESDROPPED mean?
EAVESDROPPED verb- To hear a conversation one is not intended to hear; to listen in.
- To listen for another organism's calls, so as to exploit them.