Basic desire for one belief (4)
I believe the answer is:
idea
'belief' is the definition.
(idea is a kind of belief)
'basic desire for one' is the wordplay.
'basic desire' becomes 'id' (Freudian term).
'for one' becomes 'ea' ('ea' is an abbreviation for each ie 'for one').
'id'+'ea'='IDEA'
(Other definitions for idea that I've seen before include "Something imagined in the mind" , "Mental impression" , "Thought; concept" , "Sudden inspiration?" , "Aim" .)