Eats in, when Des is out (5)
'eats' is the definition.
(dining is a kind of eating)
'in when des is out' is the wordplay.
'when' becomes 'e' (I've seen this in other clues).
'des' becomes 'ds' (I can't justify this - if you can you should give a lot more credence to this answer).
'is out' indicates putting letters inside.
'ine' put into 'ds' is 'DINES'.
Can you help me to learn more?
(Another definition for dines that I've seen is "Eats".)