Member for Portsmouth North, non-U, sarcastic (7)
I believe the answer is:
mordant
'sarcastic' is the definition.
(similar in meaning)
'member for portsmouth north' is the wordplay.
'member' becomes 'ant' (I have seen 'Member of colony' mean 'ant' so perhaps 'member' could also mean 'ant').
'for' is a charade indicator (letters next to each other) (I've seen this in other clues).
'portsmouth north' becomes 'mord' (I can't justify this - if you can you should believe this answer much more).
'ant' put after 'mord' is 'MORDANT'.
'non-u' is the link.
This may not be right. It may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for mordant that I've seen before include "Acerbic" , "Biting, scathing" , "Waspish" , "Bitingly sarcastic" , "(Of wit) sharp, critical" .)