Thinner (6)
'thinner' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for sparser that I've seen before include "not so common" , "Less thick" .)
'thinner' is the definition.
('lean' is associated in meaning with 'thin')
This is all the clue.
(Other definitions for leaner that I've seen before include "less bulky" , "Thinner, with less fat" , "less productive" , "less corpulent?" , "One with an inclination" .)