Fat pig died in lively annual celebration (9,3)
I believe the answer is:
groundhog day
'lively annual celebration' is the definition.
Although both the answer and definition are singular nouns, I can't understand how they can define each other.
'fat pig died in' is the wordplay.
'fat' becomes 'round' (round can mean fat or plump).
'pig' becomes 'hog' (synonyms).
'died' becomes 'gday' (I can't explain this - if you can you should give a lot more credence to this answer).
'in' indicates putting letters inside.
'round'+'hog'='roundhog'
'roundhog' placed within 'gday' is 'GROUNDHOG DAY'.
Can you help me to learn more?
(Other definitions for groundhog day that I've seen before include "Picture" , "Situation in which events are continually repeated" .)