Sent us adrift at nightfall (6)
I believe the answer is:
sunset
'nightfall' is the definition.
(thesaurus)
'sent us adrift' is the wordplay.
'adrift' indicates anagramming the letters.
'sent'+'us'='sentus'
'sentus' with letters rearranged gives 'SUNSET'.
'at' acts as a link.
(Other definitions for sunset that I've seen before include "End of the day in terms of light" , "This in west" , "End of the day - maybe a bright one" , "brilliant decline?" , "US nest (anag.)" .)