Pop group among 150 supported by a Jewish system of belief (7)
I believe the answer is:
cabbala
'jewish system of belief' is the definition.
(mystical tradition in Judaism)
'pop group among 150 supported by a' is the wordplay.
'pop group' becomes 'abba' (Swedish pop band).
'among' means one lot of letters goes inside another (inserted letters are placed among others).
'150' becomes 'CL' (Roman numerals).
'supported by' means one lot of letters go next to another (in a down clue, some letters hold up others).
'abba' placed within 'cl' is 'cabbal'.
'cabbal'+'a'='CABBALA'
(Other definitions for cabbala that I've seen before include "Mystical Jewish Bible interpretation" , "special scripture reading" , "Jewish mysticism" , "Jewish mystical tradition" .)