Smack bottom of scoundrel during argument (5)
I believe the answer is:
splat
'smack' is the definition.
(thesaurus)
'bottom of scoundrel during argument' is the wordplay.
'bottom of' suggests the final letters.
'during' indicates putting letters inside.
'argument' becomes 'spat' (a spat is a disagreement or argument).
The final letter of 'scoundrel' is 'l'.
'l' put within 'spat' is 'SPLAT'.
(Other definitions for splat that I've seen before include "Wet slapping sound" , "Noise on landing" , "Sound of a soft wet landing" , "amusing noise" , "Noise of soft wet impact; part of a chair back" .)