What does the cryptic crossword clue "Build-up of runs — they may retain the Ashes (4)" mean?

The definition is "Ashes". The wordplay uses an anagram. anagram of RUNS = URNS; definition: "Ashes" The answer is URNS.

Build-up of runs — they may retain the Ashes (4)

Guardian Cryptic #25377 | 26 Across | 2011-07-18 flower