Fear Gypsies dropping back a short distance (8)
I believe the answer is:
angstrom
'a short distance' is the definition.
(unit of length)
'fear gypsies dropping back' is the wordplay.
'fear' becomes 'angst'.
'gypsies' becomes 'roma'.
'dropping back' means to remove the last letter.
'roma' with its last letter taken off is 'rom'.
'angst'+'rom'='ANGSTROM'
(Other definitions for angstrom that I've seen before include "Wavelength produced" , "Unit in which light wavelengths are measured" , "Rabbit" , "scientific unit" , "A small measure" .)