Reject retreat, the last to stay (4)
I believe the answer is:
deny
'reject' is the definition.
(thesaurus)
'retreat the last to stay' is the wordplay.
'retreat' becomes 'den'.
'the last to' indicates one should take the final letters.
The last letter of 'stay' is 'y'.
'den'+'y'='DENY'
(Other definitions for deny that I've seen before include "Refuse to accept" , "Refuse to acknowledge" , "Declare untrue" , "Gainsay" , "contest" .)