Filthy lucre starts to tempt your mercilessness (7)
I believe the answer is:
cruelty
'mercilessness' is the definition.
(synonyms)
'filthy lucre starts to tempt your' is the wordplay.
'filthy' is an anagram indicator (messy letters).
'starts to' suggests taking the first letters.
The first letters of 'tempt your' is 'ty'.
'lucre' anagrammed gives 'cruel'.
'cruel'+'ty'='CRUELTY'
(Other definitions for cruelty that I've seen before include "Extreme unkindness" , "Abuse" , "Cutlery used in sadistic act" , "Inhumanity" , "Deliberate infliction of pain" .)