The responsible group a chap has to check out (6)
I believe the answer is:
senate
'the responsible group' is the definition.
The answer and definition can be both groups as well as being singular nouns.
Maybe they are linked in a way I don't understand?
'a chap has to check out' is the wordplay.
'a chap has' becomes 'nat' (I am not sure about this - if you are sure you should believe this answer much more).
'to check' becomes 'see' ('see' can be a synonym of 'check').
'out' is an insertion indicator.
'nat' placed into 'see' is 'SENATE'.
Can you help me to learn more?
(Other definitions for senate that I've seen before include "Political body" , "Upper house of the US Congress" , "Governors here" , "Upper assembly of US" , "Roman state council" .)