Let oboes become out of date (8)
I believe the answer is:
obsolete
'out of date' is the definition.
(I've seen this before)
'let oboes become' is the wordplay.
'become' indicates anagramming the letters.
'let'+'oboes'='letoboes'
'letoboes' with letters rearranged gives 'OBSOLETE'.
(Other definitions for obsolete that I've seen before include "Old hat" , "Boots eel away, being out of date" , "No longer produced or used" , "Disused" , "Dead" .)