By one? Don't talk rot! (6)
I believe the answer is:
perish
'rot' is the definition.
(thesaurus)
'by one? don't talk' is the wordplay.
'by' becomes 'per' (I've seen this in another clue).
'one?' becomes 'I' (Roman numeral).
'don't talk' becomes 'sh' ('sh' is used to tell someone to be quiet).
'per'+'i'+'sh'='PERISH'
(Other definitions for perish that I've seen before include "Shuffle off this mortal coil" , "Pass away" , "fall" , "Die - rot" , "Wither or die" .)