Tortoiseshell initially spots litter (5)
I believe the answer is:
trash
'litter' is the definition.
(I've seen this before)
'tortoiseshell initially spots' is the wordplay.
'initially' suggests taking the first letters.
'spots' becomes 'rash' (as in skin irritation).
The first letter of 'tortoiseshell' is 't'.
't'+'rash'='TRASH'
(Other definitions for trash that I've seen before include "Rubbish - destroy" , "Vandalise" , "Rubbish, worthless stuff" , "Junk" , "Refuse, waste" .)