Dim, cold and ultimately dismal: best left unfinished (9)
I believe the answer is:
candlelit
'dim' is the definition.
(lit only be candles)
'cold and ultimately dismal best left unfinished' is the wordplay.
'cold' becomes 'c' (eg on taps).
'ultimately' says to take the final letters.
'best' becomes 'elite' (elite can mean the very best of a group).
'left unfinished' means to remove the last letter.
The final letter of 'dismal' is 'l'.
'elite' with its final letter taken away is 'elit'.
'c'+'and'+'l'+'elit'='CANDLELIT'
(Other definitions for candlelit that I've seen before include "Dimly illuminated" , "With dim illumination" , "romantic setting" , "dim but romantic?" , "Softer ambient setting" .)