How do I solve 6 down in Times Cryptic 29556?

Question

Accepted Answer

6 down reads "One speed if short of power" (3 letters); the answer is ACE. 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.

One speed if short of power (3)

6D — One speed if short of power (3) — ACE