The most common and most likely answer to this crossword clue is the 7 letter word BO-PEEP
I've seen this crossword clue in Financial Times
B
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More clues leading to the result BO-PEEP
Buddy explains!
The straight part
We know that "Game/flockmistress who’s slack" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case game/flockmistress who’s slack = bo-peep.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 BO-PEEP mean?
BO-PEEP noun- The act of looking out suddenly, as from behind a screen, so as to startle someone (as by children in play), or of looking out and drawing suddenly back, as if frightened.
- (British Virgin Islands) Any whistling frog.
- To play the game of bo-peep; to peek out suddenly.