Obliterate stale air aroma pets cause at last (5)
I believe the answer is:
erase
'obliterate' is the definition.
(I've seen this before)
'stale air aroma pets cause at last' is the wordplay.
'at last' suggests the final letters.
The last letters of 'stale air aroma pets cause' are 'ERASE'.
(Other definitions for erase that I've seen before include "Eliminate" , "Obliterate" , "Rub out, cancel" , "Clear" , "Remove, wipe out" .)