Hard cheese? No thanks, when eating a vegetable! (7)
I believe the answer is:
haricot
'vegetable' is the definition.
(haricot is a kind of vegetable)
'hard cheese? no thanks' is the wordplay.
'hard' becomes 'ha' (I can't justify this - if you can you should give a lot more credence to this answer).
'cheese?' becomes 'ricotta' (ricotta is a type of cheese).
'no' suggests deleting specific letters.
'thanks' becomes 'ta' (UK informal version of 'thank you').
'ricotta' with 'ta' removed is 'ricot'.
'ha'+'ricot'='HARICOT'
'when eating a' acts as a link.
This may not be correct. Some or all of it may be part of another bit of the clue.
(Other definitions for haricot that I've seen before include "Variety of bean" , "French bean" , "Vegetable" , "kind of ragout" , "Type of bean in a chariot" .)