Junk aircraft model set up and working (8)
I believe the answer is:
jettison
'junk' is the definition.
(thesaurus)
'aircraft model set up and working' is the wordplay.
'aircraft' becomes 'jet' (jet is a kind of aircraft).
'model' becomes 'sit' (e.g. for a painting).
'set up' shows that the letters should be reversed in order (in a down clue, letters go up).
'and' is a charade indicator (letters next to each other).
'working' becomes 'on' (e.g. a machine is working or on).
'sit' reversed gives 'tis'.
'jet'+'tis'+'on'='JETTISON'
(Other definitions for jettison that I've seen before include "Form of motor racing" , "Deliberately dump overboard" , "Ditch (cargo)" , "Abandon or throw overboard" , "Throw overboard or discard unwanted material" .)