Grown-up and dull, rue the transformation (6)
I believe the answer is:
mature
'grown-up' is the definition.
(I've seen this before)
'dull rue the transformation' is the wordplay.
'dull' becomes 'mat' (mat as opposed to glossy).
'the transformation' indicates an anagram.
'rue' anagrammed gives 'ure'.
'mat'+'ure'='MATURE'
'and' acts as a link.
(Other definitions for mature that I've seen before include "Fully grown and adult" , "Grow older" , "Ripen or grow up" , "Old film star" , "reach the end of term" .)