It's dull - rue becoming an adult (6)
I believe the answer is:
mature
'adult' is the definition.
(similar in meaning)
'it's dull rue becoming' is the wordplay.
'it's dull' becomes 'mat' (mat as opposed to glossy).
'becoming' is an anagram indicator.
'rue' anagrammed gives 'ure'.
'mat'+'ure'='MATURE'
'an' is the link.
(Other definitions for mature that I've seen before include "of older years" , "Full-grown - ripe" , "Ripe or fully grown" , "due to be paid" , "Victor, old player" .)