Subsequent change, reversing Article 50 (5)
I believe the answer is:
later
'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).
'reta'+'l'='retal'
'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.
Can you help me to learn more?
(Other definitions for later that I've seen before include "At some time afterwards" , "Tardier" , "Alert (anag.)" , "there's more behind" , "More recent" .)