The most common and most likely answer to this crossword clue is the 7 letter word TAPPING
I've seen this crossword clue in The Guardian
T
A
P
P
I
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G
Buddy explains!
The straight part
We know that "Secretly listening to" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case secretly listening to = tapping.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 TAPPING mean?
TAPPING verb- To furnish with taps.
- To draw off liquid from a vessel.
- To deplete, especially of a liquid via a tap; to tap out.
- To exploit.
- To place a listening or recording device on a telephone or wired connection.
- To intercept a communication without authority.
- To cut an internal screw thread.
- To turn or flip a card or playing piece to remind players that it has already been used that turn (by analogy to "tapping," in the sense of drawing on to the point of temporary exhaustion, the resources or abilities represented by the card).
- To cadge, borrow or beg.
- To drain off fluid by paracentesis.
- To advance someone for a post or job, or for membership of a club.
- To strike lightly.
- To touch one's finger, foot, or other body parts on a surface (usually) repeatedly.
- To make a sharp noise.
- To operate an electronic device (e.g. a mobile phone) by tapping a specific place on its (capacitive or other) touch screen.
- To designate for some duty or for membership, as in 'a tap on the shoulder'.
- To have sexual intercourse with.
- To submit to an opponent by tapping one's hand repeatedly.
- To force (an opponent) to submit.
- To put a new sole or heel on.
- An act of making a light hit or strike against something.
- A guitar technique in which the strings are tapped against the fingerboard
- The process by which a resource is tapped or exploited.