How do I solve 30 across in Daily Telegraph Cryptic 31256?

Question

Accepted Answer

30 across reads "For example, Gilbert & Sullivan opera once almost ruined school event" (3-3-5,4 letters); the answer is EGG-AND-SPOON RACE. It's a deletion clue: start with a word the clue points to, then drop one or more letters. Indicators include 'endlessly' (lose the last letter), 'beheaded' (lose the first), or 'heartlessly' (lose the middle). The full step-by-step breakdown is on this page.

For example, Gilbert & Sullivan opera once almost ruined school event (3-3-5,4)

30A — For example, Gilbert & Sullivan opera once almost ruined school event (3-3-5,4) — EGG-AND-SPOON RACE