Works in molecular thermodynamics (3)
I believe the answer is:
art
'works' is the definition.
(I've seen this before)
'in molecular thermodynamics' is the wordplay.
'in' says the answer is hidden in the clue.
'ART' is hidden within 'molecular thermodynamics'.
(Other definitions for art that I've seen before include "are old-fashioned" , "Drawing" , "Works of creative imagination" , "Trickery" , "Work such as painting or sculpture" .)