More lean; paint additive (7)
I believe the answer is:
thinner
This clue is a double definition.
'more lean' is the first definition.
('thin' is associated in meaning with 'lean')
'paint additive' is the second definition.
(I've seen this in another clue)
(Other definitions for thinner that I've seen before include "Leaner" , "Skinnier" , "Slenderer" , "Less fat" , "Finer; turps, e.g." .)