How do I solve 2 down in Times Cryptic 29453?

Question

Accepted Answer

2 down reads "Crooked old mater — which strongly suggests criminal is mum" (6 letters); the answer is OMERTA. 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.

Crooked old mater — which strongly suggests criminal is mum (6)

2D — Crooked old mater — which strongly suggests criminal is mum (6) — OMERTA