Char about to get less fat (7)
I believe the answer is:
cleaner
'char' is the definition.
(thesaurus)
'about to get less fat' is the wordplay.
'about' becomes 'c' (abbreviation for 'circa').
'to get' is a charade indicator (letters next to each other).
'less fat' becomes 'leaner' ('lean' can be the opposite of 'fat' so perhaps 'leaner' is reasonable for 'less fat').
'c'+'leaner'='CLEANER'
(Other definitions for cleaner that I've seen before include "One who dusts or vacuums" , "Charlady" , "More unblemished - charwoman" , "More sanitary" , "Worker" .)