Food occasionally dreary? This soup's rich (7)
I believe the answer is:
chowder
'this soup's rich' is the definition.
'chowder' can be an answer for 'soup's' (chowder is a kind of soup). I'm not sure about the remainder of the definition.
'food occasionally dreary?' is the wordplay.
'food' becomes 'chow' (chow can informally mean food).
'occasionally' means one should take alternating letters.
The alternate letters of 'dreary' are 'der'.
'chow'+'der'='CHOWDER'
Can you help me to learn more?
(Other definitions for chowder that I've seen before include "Rich seafood soup" , "Clam stew, soup" , "Thick soup containing clams or fish" , "Thick fish stew" , "Thick seafood soup" .)