A waltz at last in gloomy darkness then a lively Polish dance (7)
I believe the answer is:
mazurka
'lively polish dance' is the definition.
(I've seen this in another clue)
'a waltz at last in gloomy darkness then a' is the wordplay.
'at last' suggests the final letters.
'in' indicates putting letters inside.
'gloomy darkness' becomes 'murk' (I've seen this before).
'then' means one lot of letters go next to another.
The last letter of 'waltz' is 'z'.
'a'+'z'='az'
'az' going within 'murk' is 'mazurk'.
'mazurk'+'a'='MAZURKA'
(Other definitions for mazurka that I've seen before include "Polish dance music" , "Lively Polish dance" , "Polish dance in triple time" , "Polish national dance" , "Polish folk dance" .)