First of children, thinner and not as dirty (7)
I believe the answer is:
cleaner
'not as dirty' is the definition.
('clean' can be the opposite of 'dirty' so perhaps 'cleaner' is reasonable for 'not as dirty')
'first of children thinner' is the wordplay.
'first of children' becomes 'c' (1st letter of 'children').
'thinner' becomes 'leaner' ('lean' can be a synonym of 'thin').
'c'+'leaner'='CLEANER'
'and' acts as a link.
(Other definitions for cleaner that I've seen before include "Less soiled" , "Maybe Daily" , "Charwoman - vacuum" , "Char, daily" , "Dirt remover" .)