Let outside old information unit (8)
I believe the answer is:
roentgen
'unit' is the definition.
(roentgen is a kind of unit)
'let outside old information' is the wordplay.
'let' becomes 'rent' (synonyms).
'outside' indicates putting letters inside.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'information' becomes 'gen' (gen can informally mean information).
'rent' going around 'o' is 'roent'.
'roent'+'gen'='ROENTGEN'
(Other definitions for roentgen that I've seen before include "German physicist" , "Measure of radiation" , "Pioneer in radiation" , "X-ray discoverer" .)