Subsequent change, reversing Article 50 (5)
I believe the answer is:
Here is my best explanation:
'subsequent' is the definition.
(similar in meaning)
'reversing article 50' is the wordplay.
'reversing' says the letters should be written in reverse.
'article' becomes 'reta' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'50' becomes 'l' (roman numeral).
'retal' backwards is 'LATER'.
'change' acts as a link.
I am not very happy about this link. It may belong to another bit of the clue.