What does the cryptic crossword clue "A device for catching people in the act? (6,3)" mean?

The definition is "A device for catching people in the act?". The wordplay uses a cryptic definition. Cryptic definition The answer is SAFETYNET.

A device for catching people in the act? (6,3)

Guardian Cryptic #27311 | 21 Across | 2017-09-25 flower