Heidegger regularly displays sharpness of mind (4)
I believe the answer is:
edge
'sharpness of mind' is the definition.
(I've seen this before)
'heidegger regularly' is the wordplay.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
The alternate letters of 'heidegger' are 'EDGE'.
'displays' acts as a link.
(Other definitions for edge that I've seen before include "Margin, extremity" , "'Margin, rim (4)'" , "Surface boundary" , "'Border, boundary, rim (4)'" , "Sharpness of mind" .)