Stops eating quickly on Sunday (5)
I believe the answer is:
fasts
'stops eating' is the definition.
Although both the answer and definition are verbs in their -s form, I cannot see how one could define the other.
'quickly on sunday' is the wordplay.
'quickly' becomes 'fast' ('fast' can be a synonym of 'quickly').
'on' says to put letters next to each other.
'sunday' becomes 's'.
'fast'+'s'='FASTS'
Can you help me to learn more?
(Other definitions for fasts that I've seen before include "Times of abstinence" , "Goes hungry" , "Abstains from food" , "Goes without food" , "Avoids food" .)