Not able to write and angry about liberal diet (10)
I believe the answer is:
illiterate
'not able to write' is the definition.
(illiterate means unable to read or write)
'angry about liberal diet' is the wordplay.
'angry' becomes 'irate' (irate can mean angry or annoyed).
'about' means one lot of letters goes inside another.
'liberal' becomes 'l' (abbreviation).
'diet' becomes 'lite' (synonyms).
'l'+'lite'='llite'
'irate' going around 'llite' is 'ILLITERATE'.
'and' acts as a link.
(Other definitions for illiterate that I've seen before include "dunce" , "uneducated person" , "one who can't read" , "Analphabetic" , "non-reader" .)