She only gained a start as an emergency but Diddums has belied her standing to claim a Group Three win at Randwick.

The filly was a $19 chance once she made the field for Saturday's 1200m sprint but has always been a favourite with her trainer, Gerald Ryan.

Diddums left the gates swiftly for Sam Clipperton and raced on the pace before drawing away to beat Prompt Response ($7) by 1-1/4 lengths, with Smart Amelia ($26) another three-quarters of a length in third.

The $2.35 favourite Raiment was never in the hunt and finished seventh, seven lengths from the winner.

"She is a good honest filly who always seems to be under the radar," Ryan said.

"It's good for owner John Cornish and I'm rapt because she's by Snitzel."

Ryan trained Snitzel and has had more success with his progeny than any other trainer.

The filly was ridden by Hong Kong-ased Sydneysider Sam Clipperton, who was in a little distress after the energy-sapping ride.

He has been battling the flu and was OK when he had rehydrated.

"It's great to come home and get a winner," Clipperton said.

"Gerald left it up to me whether I led or took a sit. I got there effortlessly and was able to rate her and she was always going to give me a good kick. We stayed hard up against the fence."

Godolphin jockey James Doyle could offer little explanation for the unplaced performance of Raiment, a dominant winner in similar heavy conditions last time out.

"She was very disappointing," he said.

"A big performance last start on this ground may have taken its toll today.

"They went slow so I improved albeit a little bit wide. The minute I straightened I was out on my feet."