Party's leader longing for witch hunt (5)
I believe the answer is:
purge
'for witch hunt' is the definition.
'party's leader longing' is the wordplay.
'leader' indicates taking the first letters.
'longing' becomes 'urge' (both can mean a desire).
The first letter of 'partys' is 'p'.
'p'+'urge'='PURGE'
(Other definitions for purge that I've seen before include "empty bowels" , "Rid organisation of undesirable members" , "pogrom" , "Get rid of unwanted elements" , "Purify, clear out" .)