How do I solve 2 down in Daily Telegraph Cryptic 31242?

Question

Accepted Answer

2 down reads "What remains of troops you heard breaching flank" (7 letters); the answer is RESIDUE. 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.

What remains of troops you heard breaching flank (7)

2D — What remains of troops you heard breaching flank (7) — RESIDUE