The phrase "a lap down" is correct and usable in written English.
It is typically used in the context of racing, indicating that a competitor has completed one fewer lap than the leader or the main group of competitors.
Example: "After the pit stop, he found himself a lap down and had to work hard to catch up to the pack."
Alternatives: "one lap behind" or "behind by a lap".
"What was I supposed to do, go a lap down?" he told reporters after the race.
Almost unnoticed, and from half a lap down, "King" Richard Petty won the race with Darrell Waltrip and AJ Foyt right on his bumper.
Both Toyotas went a lap down after going off as did the No20 Porsche of Mark Webber, Brendon Hartley and Timo Bernhard.
The unfortunate timing cost Earnhardt dearly; he slipped a lap down and never got back in the mix, though all the trouble at the end boosted him to 15th.
Keselowski did make it hard for Busch to pass him late in the race, when Keselowski was fighting not to go a lap down, but Busch made a clean move around him in his No. 18 Joe Gibbs Racing Toyota after several attempts.
The other Audi of McNish and Kristensen in the diesel-powered non-hybrid R18 Ultra had seen their challenge thwarted by a slow puncture towards the end of the second hour that was compounded when they were forced a lap down during a safety car period.
I have never understood how someone can celebrate a victory like that [Brazil] one lap down and winning after someone else has been disqualified".
He was held on pit road for a lap, once again dropping from the top five to 35th, one lap down with 77 to go.
Both Alonso and Jenson Button, who finished 16th and whose future remains unclear, finished one lap down on Hamilton.
At this stage Mercedes may still be a little faster but last season's half-a-second a lap may be down to two or three-tenths.
The most striking was the much higher likelihood of younger injured children having been on a lap when going down the slide and having suffered a lower extremity injury, in particular, a lower leg/ankle fracture.
