One eating a piece of celery with too much noise (5)
'one eating' is the definition.
(I've seen this before)
'a piece of celery with too much noise' is the wordplay.
'a piece of celery' becomes 'er' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'with' means one lot of letters go next to another.
'too much noise' becomes 'din' (I've seen this before).
'er' after 'din' is 'DINER'.
Can you help me to learn more?
(Other definitions for diner that I've seen before include "American restaurant", "Restaurant client", "Restaurant customer", "Eater", "One eating a meal".)