The writer thus to drink a substance (7)
I believe the answer is:
isotope
'a substance' is the definition.
(isotope is a kind of substance)
'the writer thus to drink' is the wordplay.
'the writer' becomes 'I' (the person who is writing these clues).
'thus' becomes 'so' (synonyms).
'to drink' becomes 'tope' (synonyms).
'i'+'so'+'tope'='ISOTOPE'
(Other definitions for isotope that I've seen before include "Atom with the same atomic number as another but with different numbers of neutrons" , "O, is poet a species of atom?" , "Oops, tie up the atom" , "elementary variation in it" , "Variant of a particular chemical element -- ie stoop (anag)" .)