Set aside letter looking into article 50 (5)
I believe the answer is:
annul
'set aside' is the definition.
(thesaurus)
'letter looking into article 50' is the wordplay.
'letter looking' becomes 'nu' (I can't justify this - if you can you should give a lot more credence to this answer).
'into' means one lot of letters goes inside another.
'article' becomes 'an' (indefinite article).
'50' becomes 'l' (roman numeral).
'an'+'l'='anl'
'nu' inserted within 'anl' is 'ANNUL'.
Can you help me to learn more?
(Other definitions for annul that I've seen before include "Cancel, make void" , "Declare (a law) invalid" , "Quash" , "Cancel officially" , "Cancel, abolish" .)