Therefore US spy agency joins Liberal Party (6)
I believe the answer is:
social
'liberal party' is the definition.
'social' can be an answer for 'party' (social can mean a party or event). I'm unsure of the 'liberal' bit.
'therefore us spy agency joins' is the wordplay.
'therefore' becomes 'so' ('so' can mean 'consequently' or 'therefore').
'us spy' becomes 'l' (I can't explain this - if you can you should believe this answer much more).
'agency' becomes 'CIA' (CIA is an example).
'joins' is a charade indicator (letters next to each other).
'l' put after 'cia' is 'cial'.
'so'+'cial'='SOCIAL'
Can you help me to learn more?
(Other definitions for social that I've seen before include "Pertaining to human relations" , "Outgoing, extroverted" , "Gregarious; a party" , "Informal gathering" , "Of society or its organisation" .)