Physicist's anxiety over memory (8)
I believe the answer is:
angstrom
'physicist's' is the definition.
(I've seen this before)
'anxiety over memory' is the wordplay.
'anxiety' becomes 'angst' (angst is a type of anxiety).
'over' says to put letters next to each other (in a down clue, some letters go over others).
'memory' becomes 'ROM' (Read-Only Memory on a computer).
'angst'+'rom'='ANGSTROM'
(Other definitions for angstrom that I've seen before include "Swedish scientist" , "Wavelength produced" , "A small measure" , "Microscopic length unit" , "Unit of wavelength measurement" .)