What does the cryptic crossword clue "He devised a constant sounding board (6)" mean?

The definition is "He devised a constant". The wordplay uses a homophone (sounds like). Sounds like [sounding] plank [board] The answer is PLANCK.

He devised a constant sounding board (6)

Guardian Cryptic #26228 | 3 Down | 2014-04-08 flower