How do I solve 21 down in Daily Telegraph Cryptic 31254?

Question

Accepted Answer

21 down reads "Last kid swimming for island group" (2,5 letters); the answer is ST KILDA. 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.

Last kid swimming for island group (2,5)

21D — Last kid swimming for island group (2,5) — ST KILDA