Less fat when solvent (7)
I believe the answer is:
thinner
I believe this clue is a double definition.
'less fat' is the first definition.
('thin' can be the opposite of 'fat' so perhaps 'thinner' is reasonable for 'less fat')
'solvent' is the second definition.
(synonyms)
'when' acts as a link.
(Other definitions for thinner that I've seen before include "More runny" , "Diluting solvent" , "less chubby" , "Finer; turps, e.g." , "More slim" .)