Star name, one adopting fashionable diet (5)
I believe the answer is:
vegan
'fashionable diet' is the definition.
'star name one adopting' is the wordplay.
'star' becomes 'vega' (Vega is an example).
'name' becomes 'n' (common abbreviation e.g. on forms).
'one adopting' says to put letters next to each other.
'vega'+'n'='VEGAN'
(Other definitions for vegan that I've seen before include "one not using animals" , "Strict vegetarian" , "One not eating animals" , "Person neither eating nor using anything animal" , "Avoider of animal products" .)