At last, AtmosClear! We offer you clean air again (5)
I believe the answer is:
rerun
'air again' is the definition.
'at last atmosclear we offer you clean' is the wordplay.
'at last' suggests the final letters.
The last letters of 'atmosclear we offer you clean' are 'RERUN'.
(Other definitions for rerun that I've seen before include "return to track" , "Show or perform again" , "Repeat of, say, a film" , "Play again" , "Another airing" .)