Runt finally spots litter (5)
I believe the answer is:
trash
'litter' is the definition.
(thesaurus)
'runt finally spots' is the wordplay.
'finally' says to take the final letters.
'spots' becomes 'rash' (I've seen this before).
The last letter of 'runt' is 't'.
't'+'rash'='TRASH'
(Other definitions for trash that I've seen before include "Vandalise" , "Worthless people; destroy (US slang)" , "Refuse; wreck" , "bull" , "Anything of poor quality" .)