Crooked looking prime minister not quite shutting up last of opposition (7)
I believe the answer is:
asquint
'crooked looking' is the definition.
'prime minister not quite shutting up last of opposition' is the wordplay.
'prime minister' becomes 'asquith' (I've seen this before).
'not quite' means to remove the last letter.
'shutting up' indicates putting letters inside.
'last of' indicates one should take the final letters.
The final letter of 'opposition' is 'n'.
'asquith' with its last letter taken away is 'asquit'.
'asquit' enclosing 'n' is 'ASQUINT'.
(Another definition for asquint that I've seen is " Glancing sideways".)