Solvent, bar in the face of ruin (7)
I believe the answer is:
thinner
'solvent' is the definition.
('thinner' can be a synonym of 'solvent')
'bar in the face of ruin' is the wordplay.
'bar' becomes 'inn' (I've seen this before**).
'in' means one lot of letters goes inside another.
'face of' indicates taking the first letters.
The first letter of 'ruin' is 'r'.
'inn' going within 'the' is 'thinne'.
'thinne'+'r'='THINNER'
(Other definitions for thinner that I've seen before include "Leaner" , "More sparse" , "Diluting agent" , "Diluting solvent" , "what one adds to paint" .)