Former parliamentary law agreement underpinning masses in uprising (5,3)
I believe the answer is:
stamp act
'former parliamentary law' is the definition.
'stamp act' can be an answer for 'law' (I've seen this before). I am not certain of the 'former parliamentary' bit.
'agreement underpinning masses in uprising' is the wordplay.
'agreement' becomes 'pact' (a pact is an agreement or deal).
'underpinning' says to put letters next to each other (in a down clue, some letters go under others).
'masses in uprising' becomes 'stam' (I am not sure about this - if you are sure you should believe this answer much more).
'pact' after 'stam' is 'STAMP ACT'.
Can you help me to learn more?
(Other definitions for stamp act that I've seen before include "Tax decree" , "piece of legislation" , "Unpopular law" .)