Extremely slender for example; that's about all (10)
I believe the answer is:
everything
'all' is the definition.
(all things)
'extremely slender for example that's about' is the wordplay.
'extremely' becomes 'very' (very can mean extremely or considerably).
'slender' becomes 'thin' (synonyms).
'for example' becomes 'e.g.' (short for 'exempli gratia', 'for example' in Latin).
'that's about' means one lot of letters goes inside another.
'very'+'thin'='verythin'
'verythin' placed inside 'eg' is 'EVERYTHING'.
(Other definitions for everything that I've seen before include "The whole shebang" , "the works" , "The whole lot" , "All items" , "great deal" .)