Not as bright, the German accepting one millimetre (6)
I believe the answer is:
dimmer
'not as bright' is the definition.
(I've seen this before)
'the german accepting one millimetre' is the wordplay.
'the german' becomes 'der' ('the' in German).
'accepting' indicates putting letters inside (inserted letters are accepted).
'one' becomes 'i' (Roman numeral).
'millimetre' becomes 'mm' (synonyms).
'i'+'mm'='imm'
'der' going around 'imm' is 'DIMMER'.
(Other definitions for dimmer that I've seen before include "Light controller" , "Not so bright or harsh" , "Electrical switch" , "Less bright" , "Fainter" .)