Girl in esoteric assembly (5)
I believe the answer is:
erica
'girl' is the definition.
'in esoteric assembly' is the wordplay.
'in' indicates the answer is hidden within the clue.
'ERICA' is hidden in the letters of 'esoteric assembly'.
(Other definitions for erica that I've seen before include "Girl's name - I care (anag)" , "found on the moors" , "Plant genus of garden heather" , "ling" , "Shrub genus; girl's name" .)