Brilliant and popular among cast (5)
I believe the answer is:
shiny
'brilliant' is the definition.
(I've seen this in another clue)
'popular among cast' is the wordplay.
'popular' becomes 'in' ('in' can mean trendy).
'among' means one lot of letters goes inside another.
'cast' becomes 'shy' (I've seen this before).
'in' put inside 'shy' is 'SHINY'.
'and' is the link.
(Other definitions for shiny that I've seen before include "Polished, reflective" , "Smooth and reflective" , "Lustrous, glossy" , "Bright" , "Smooth and polished" .)