Fool joining a cult: not the first — mug! (7)
I believe the answer is:
assault
'mug' is the definition.
(mug can mean to attack and rob someone)
'fool joining a cult not the first' is the wordplay.
'fool' becomes 'ass' (both can mean a stupid person).
'joining' is a charade indicator (letters next to each other).
'not the first' means to remove the first letter.
'cult' with its initial letter taken off is 'ult'.
'ass'+'a'+'ult'='ASSAULT'
(Other definitions for assault that I've seen before include "Sort of course" , "Attack physically" , "Storm" , "Attack associated with" , "offensive" .)