Reveal journalist supporting defence ministry to be obsolete (8)
I believe the answer is:
outmoded
'be obsolete' is the definition.
('outmoded' can be a synonym of 'obsolete')
'reveal journalist supporting defence ministry' is the wordplay.
'reveal' becomes 'out' (outing is a kind of revealing).
'journalist' becomes 'ed' (short for editor).
'supporting' is a charade indicator (letters next to each other) (in a down clue, some letters go below others).
'defence ministry' becomes 'mod' (I've seen this in other clues).
'ed' put after 'mod' is 'moded'.
'out'+'moded'='OUTMODED'
'to' acts as a link.
(Other definitions for outmoded that I've seen before include "No longer in fashion" , "Old-fashioned" , "Dated" , "No longer fashionable" .)