Popular plan, besides, crazy but essential (13)
I believe the answer is:
indispensable
'essential' is the definition.
('indispensable' is associated in meaning with 'essential')
'popular plan besides crazy' is the wordplay.
'popular' becomes 'in'.
'crazy' indicates anagramming the letters.
'plan'+'besides'='planbesides'
'planbesides' anagrammed gives 'dispensable'.
'in'+'dispensable'='INDISPENSABLE'
'but' acts as a link.
(Other definitions for indispensable that I've seen before include "Essential" , "Absolutely necessary" , "Vitally necessary" .)