Solvent, having lost weight (7)

I believe the answer is:

thinner

I'm a little stuck... Click here to teach me more about this clue!

'solvent' is the definition.
(synonyms)

I cannot understand how the rest of the clue works.

(Other definitions for thinner that I've seen before include "Slenderer" , "Volatile liquid" , "Skinnier" , "what one adds to paint" , "More flimsy" .)

I've seen this clue in The Daily Express.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.