Science of a particle with variable (7)
I believe the answer is:
anatomy
'science' is the definition.
(anatomy is a kind of science)
'a particle with variable' is the wordplay.
'a' becomes 'an'.
'particle' becomes 'atom' (synonyms).
'with' is a charade indicator (letters next to each other).
'variable' becomes 'y' (y may represent a variable in algebra).
'an'+'atom'+'y'='ANATOMY'
'of' is the link.
(Other definitions for anatomy that I've seen before include "Bodily structure of animal or plant" , "(Science of) physical structure of animals or plants" , "The scientific study of bodily structures" , "Branch of biology" , "mummy?" .)