Pop star crazy about sodium (7)
I believe the answer is:
madonna
'pop star' is the definition.
(I've seen this before)
'crazy about sodium' is the wordplay.
'crazy' becomes 'mad' (synonyms).
'about' becomes 'on' (as in a book 'on' a certain topic).
'sodium' becomes 'NA' (Na is the chemical symbol for sodium).
'mad'+'on'+'na'='MADONNA'
(Other definitions for madonna that I've seen before include "Guy's girl" , "Like a Virgin singer" , "Representation of the Virgin Mary" , "Picture or statue of the Virgin Mary" , "Religious art subject - pop singer, b. 1958" .)