What does the cryptic crossword clue "Old debugger working to pick up a mark of reference (6,6)" mean?

The wordplay uses an anagram. anagram of OLD + DEBUGGER + A = DOUBLEDAGGER The answer is DOUBLEDAGGER.

Old debugger working to pick up a mark of reference (6,6)

Guardian Cryptic #29539 | 14 Across | 2024-11-13 flower