Because a gloss finish can be nice? (5)
I believe the answer is:
since
'because' is the definition.
('since' can mean 'because')
'a gloss finish can be nice?' is the wordplay.
'finish' indicates one should take the final letters.
'can be' indicates anagramming the letters.
The final letter of 'gloss' is 's'.
'nice' anagrammed gives 'ince'.
's'+'ince'='SINCE'
(Other definitions for since that I've seen before include "From that time one" , "in the past" , "Seeing that" , "Because; ago" .)