Judge supports embargo on old instrument (5)
I believe the answer is:
banjo
'instrument' is the definition.
(I know that banjo is a type of stringed instrument)
'judge supports embargo on old' is the wordplay.
'judge' becomes 'J' (abbreviation for judge).
'supports' says to put letters next to each other (bottom letters provide a support for the top ones).
'embargo' becomes 'ban' (a ban on trade).
'on' means one lot of letters go next to another (in a down clue, letters appear on others).
'old' becomes 'o' (abbreviation).
'j' after 'ban' is 'banj'.
'banj'+'o'='BANJO'
(Other definitions for banjo that I've seen before include "something that's played on" , "Formby's instrument" , "Stringed instrument with circular body" , "Guitar-like instrument" , "Uke's relative" .)