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' indicates an anagram.
'rue' with letters rearranged gives 'ure'.
'mat'+'ure'='MATURE'
'an' acts as a 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" .)