Compound of lethal arsenic returned by the writer nevertheless (3,3,4)
I believe the answer is:
all the same
'nevertheless' is the definition.
(synonyms)
'compound of lethal arsenic returned by the writer' is the wordplay.
'compound of' indicates anagramming the letters (I've seen this in other clues).
'arsenic' becomes 'as' (As is the chemical symbol for arsenic).
'returned' shows that the letters should be reversed in order.
'by' is a charade indicator (letters next to each other).
'the writer' becomes 'me' (the person who is writing these clues).
'as' in reverse letter order is 'sa'.
'lethal' is an anagram of 'allthe'.
'allthe'+'sa'+'me'='ALL THE SAME'
(Other definitions for all the same that I've seen before include "Nevertheless - identical" , "like clones?" , "In any event" , "in spite of this" , "everyone is in uniform" .)