How do I solve 6 down in Daily Telegraph Cryptic 31236?

Question

Accepted Answer

6 down reads "Acting detective let his gun off" (9 letters); the answer is SLEUTHING. It's an anagram clue: the answer is a rearrangement of letters supplied by the clue itself. Look for an indicator like 'mixed', 'broken', 'cooked', or 'wild', and find a phrase whose letters add up to the answer's length. The full step-by-step breakdown is on this page.

Acting detective let his gun off (9)

6D — Acting detective let his gun off (9) — SLEUTHING