Micromillimetre — a term none read wrongly (9)
I believe the answer is:
nanometre
'micromillimetre' is the definition.
(I know that micromillimetre can be written as nanometre)
'a term none read wrongly' is the wordplay.
'read wrongly' is an anagram indicator.
'a'+'term'+'none'='atermnone'
'atermnone' is an anagram of 'NANOMETRE'.
(Other definitions for nanometre that I've seen before include "small unit" , "tiny distance" , "short distance" , "Very, very short length" , "Some tiny measure" .)