Derision cast about politician behind bars (10)
I believe the answer is:
imprisoned
'behind bars' is the definition.
(I've seen this before)
'derision cast about politician' is the wordplay.
'cast' indicates an anagram.
'about' means one lot of letters goes inside another.
'politician' becomes 'mp'.
'derision' is an anagram of 'irisoned'.
'irisoned' enclosing 'mp' is 'IMPRISONED'.
(Other definitions for imprisoned that I've seen before include "Put away" , "Locked up" , "behind bars" , "in jail" .)