Shy in disposition, but bright (5)
I believe the answer is:
shiny
'bright' is the definition.
(I've seen this before)
'shy in disposition' is the wordplay.
'disposition' indicates an anagram.
'shy'+'in'='shyin'
'shyin' with letters rearranged gives 'SHINY'.
'but' acts as a link.
(Other definitions for shiny that I've seen before include "Smooth and reflective" , "Smooth and polished" , "Polished, reflective" , "Polished and reflecting light" , "Lustrous, glossy" .)