Something rude to do is eating very fast (6)
I believe the answer is:
starve
'fast' is the definition.
(to fast is to refrain from eating food)
'something rude to do is eating very' is the wordplay.
I cannot quite understand how this works, but
'do' could be 'star' (starring is a kind of doing) and 'star' is found in the answer.
'very' could be 'v' (abbreviation) and 'v' is found in the answer.
A single letter 'e' remains which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for starve that I've seen before include "Suffer great hunger" , "Suffer from want of food" , "Deprive of sustenance" , "Deprive of food" , "Die from lack of food" .)