Old instrument a British journal’s taken up (5)
I believe the answer is:
gamba
'old instrument' is the definition.
(I've seen this before)
'a british journal's taken up' is the wordplay.
'british' becomes 'b' (abbreviation e.g. in 'BBC').
'journal's taken' becomes 'mag' (I can't justify this - if you can you should give a lot more credence to this answer).
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'a'+'b'+'mag'='abmag'
'abmag' backwards is 'GAMBA'.
Can you help me to learn more?
(Another definition for gamba that I've seen is " Old stringed instrument".)