Pacify Liberals mobbing leader of Unionists (4)
I believe the answer is:
lull
'pacify' is the definition.
(thesaurus)
I cannot understand how the rest of the clue works.
(Other definitions for lull that I've seen before include "Put to sleep - respite" , "Soothe; quiet interval" , "Cessation of activity (before a storm?)" , "Subside" , "Soothe or send to sleep" .)