If duller, a gloss finish is poorer (6)

I believe the answer is:

slower

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'if duller a gloss' is the definition.
I can't tell whether this defines the answer.

'finish is poorer' is the wordplay.
'finish' means to take half the letters.
'poorer' becomes 'lower' ('low' can be similar in meaning to 'poor').
'is' halved is 's'.
's'+'lower'='SLOWER'

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(Other definitions for slower that I've seen before include "Being less intelligent" , "taking longer" , "Less able to understand" , "Not so fast!" .)

I've seen this clue in the Evening Standard.

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