Unusually strong mother back about electromagnetic wavelength unit (8)
I believe the answer is:
angstrom
'electromagnetic wavelength unit' is the definition.
(the angstrom is used to measure wavelengths)
'unusually strong mother back about' is the wordplay.
'unusually' indicates anagramming the letters.
'mother' becomes 'ma' (ma can mean mother).
'back' says the letters should be written backwards.
'about' means one lot of letters goes inside another.
'ma' backwards is 'am'.
'strong' with letters rearranged gives 'ngstro'.
'ngstro' placed within 'am' is 'ANGSTROM'.
(Other definitions for angstrom that I've seen before include "scientific unit" , "Wavelength produced" , "Unit of wavelength measurement" , "0.0001 micron" , "Tiny unit of length used in Physics" .)