First of contestants, 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 contestants thinner' is the wordplay.
'first of contestants' becomes 'c' (1st letter of 'contestants').
'thinner' becomes 'leaner' ('lean' can be a synonym of 'thin').
'c'+'leaner'='CLEANER'
'and' is the link.
(Other definitions for cleaner that I've seen before include "less sleazy?" , "Char; less dirty" , "More sanitary" , "Daily help" , "Maybe Daily" .)