According to this, 50 all out! (4)
I believe the answer is:
plan
'according to this 50 all out' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'according to this 50 all out' is the wordplay.
'according to' becomes 'p' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'this 50' becomes 'l' ('L' can be a synonym of '50').
'all' becomes 'any' ('any' can be similar in meaning to 'all').
'out' means to remove the last letter.
'any' with its final letter taken away is 'an'.
'p'+'l'+'an'='PLAN'
Can you help me to learn more?
(Other definitions for plan that I've seen before include "Large-scale map" , "Schedule" , "Map; intention" , "propose" , "Devise a way to achieve" .)