German physicist more intelligent than he sounds? (6)

I believe the answer is:

planck

I'm a little stuck... Click here to teach me more about this clue!

'german physicist' is the definition.
(I've seen this before)

I can't explain the rest of the clue.

(Other definitions for planck that I've seen before include "Max ---, quantum theorist" , "German physicist giving name to a constant" , "Max --, German scientist, gave name to a constant" , "German physicist, initiator of quantum theory, d. 1947" .)

I've seen this clue in The Guardian.

Want a hint initially instead of a full solution? Install my app