Toxic substance covered Lima shoreline (6)
I believe the answer is:
bleach
'toxic substance' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'covered lima shoreline' is the wordplay.
I cannot really see how this works, but
'lima' could be 'l' (phonetic alphabet: alpha, bravo, charlie etc.) and 'l' is located in the answer.
'shoreline' could be 'beach' (I have seen 'Accessible shoreline ' mean 'beach' so perhaps 'shoreline' could also mean 'beach') and 'beach' is found within the leftover letters.
No letters remain.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for bleach that I've seen before include "Get lighter" , "Make colourless" , "Go white" , "Strong cleaner" , "Sterilising whitener" .)