Bar coated with the right solvent (7)
I believe the answer is:
thinner
'solvent' is the definition.
(synonyms)
'bar coated with the right' is the wordplay.
'bar' becomes 'inn' (I've seen this before).
'coated with' means one lot of letters goes inside another.
'right' becomes 'R' (common abbreviation).
'inn' inserted inside 'the' is 'thinne'.
'thinne'+'r'='THINNER'
(Other definitions for thinner that I've seen before include "More slim" , "Diluting solvent" , "More sparse" , "Less fat" , "Volatile liquid" .)