A tense assembly or governing body (6)
I believe the answer is:
senate
'governing body' is the definition.
(type of governing body)
'a tense assembly' is the wordplay.
'assembly' is an anagram indicator.
'a'+'tense'='atense'
'atense' with letters rearranged gives 'SENATE'.
'or' acts as a link.
(Other definitions for senate that I've seen before include "Upper house of the US Congress" , "Second house of the Oireachtas" , "The upper house in parliament or oireachtas" , "Legislative body" , "State assembly" .)